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An inter-collegiate scuba diving club composed of students from Izu University and Oumi Women's University in the city of Izu in Shizuoka Prefecture, Peek-a-Boo, or PaB for short, uses the titular "Grand Blue" diving goods shop as their meeting place for club activities and the main source for equipment and diving services... when they're not busy partying hard, getting absolutely smashed, and/or running around in varying states of undress.

    As a Club 

  • Badass Armfold: Kotobuki and Tokita tend to fold their arms every now and then while talking.
  • Bunny-Ears Lawyer: The seniors may spend their time drinking and partying, but when it comes to anything related to actual diving, they are pretty good at it.
  • The Corruption: Played for Laughs with the club's influence on Iori and Kouhei, who were initially weirded out by their seniors' tendencies, but has since began to participate. It's implied their tolerance for alcohol has also increased because of how much they've had to drink since joining the club. It also crosses over into Corrupt the Cutie territory when Aina joins in.
  • Enraged by Idiocy: Chisa and Aina will react with scorn whenever Peek-A-Boo starts with their usual antics.
  • Everyone Has Standards:
    • Despite the abundant alcohol consumption, frequent nudity and sex discussions, the club is disturbed by Nanaka's unhealthy affection for Chisa.
    • Sure, they're party animals, but when when it comes to diving, that's no time for jokes. The seniors always make sure to let their underclassmen know that even when it's beautiful, being underwater and making a mistake there could escalate into something dangerous.
    • For all their raunchy behavior, the boys make it a point to never let any creep come even remotely close to any of the girls. Even the tiniest bit of ill intent is bound to invite their righteous wrath.
  • Heterosexual Life-Partners: Ryuujirou and Toki are always together and get along well.
  • Meaningful Name: "Peek-a-Boo" is a not-so-subtle nod to the nudism rampant with its male majority; even its portmanteau name "PaB" may be read as "pub", befitting its culture of alcoholism.
  • Naked People Are Funny: A major Running Gag throughout the series is the boys' tendency to run around in varying states of undress, so much, that they somehow incorporate nudity in completely unrelated situations, and it somehow has bled outside the activities of the club to the point of exhibiting nonchalant exhibitionism outside common societal norms.
  • Not Good with People: With the exception of Iori, the other three freshmen lack social skills and rarely to talk to people who they aren't close with:
    • In addition to having hobbies not many people his age has, Kohei is also afraid of strangers, often stammers whenever he meets someone he doesn't knows (or forgets).
    • Chisa rarely has any friends outside of the club and spends more time in anything related to diving, which got her chasized by her mother.
    • In order to fit better among her peers, Aina puts on heavy make up and tries on clothes that doesn't match her.
  • Oblivious to Love:
    • Iori is unaware of just how much of a ladykiller he is, having already attracted the attention of Aina, Sakurako, and even Naomi.
    • Chisa doesn't seem to realize Nanaka loves her more than just a sister.
  • The Pirates Who Don't Do Anything: Despite its ostensible activities, most members of Peek-a-Boo spend more time drinking, playing games, and walking around half- or fully naked. It's a shock for Iori and Kouhei when the club actually does something diving-related.
  • Senpai Kohai: Freshmen Iori, Chisa, Kouhei and Aina to third-years Ryuujirou, Shinji and Azusa (as well as college graduate Nanaka).
  • This Is Wrong on So Many Levels: Outsiders are often bewildered at the club's sheer alcoholism, and how much it comes up whenever they interact with a club member (usually Iori or Kouhei).
  • Those Two Guys: Ryuujirou and Shinji are usually seen together.
  • True Companions:
    • Despite their vastly differing personalities and frequent loggerheads, the members of Peek-a-Boo share a great chemistry and teamwork forged through college life, drinking sessions, and the fact that scuba diving is an activity where safety is of paramount importance, and thus is best done as a team. During the Okinawa trip Aina even contemplates on how much of an improvement Peek-a-Boo is, quirks and all, compared to her former club, the Tinkerbell tennis team, where everyone else, even Captain Kudou, shunned her for her awkward attempts at socialization.
    • It's stated by Azusa that to achieve the level of mutual trust that they show among themselves, the three main senpai and Nanaka had a long story between them before the kouhai came into the fold. As Asuza states, it's not like they were sitting on their hands the whole time... and if her words are to be taken at face value, there was some saucy stuff.
  • Vitriolic Best Buds: Iori and Kouhei fight about everything, yet despite their vastly different lifestyles, their personalities are seen as similar and spend most of their time together, partly due to having been inducted into the club at around the same time, and partly out of their chagrin over their upperclassmen's shenanigans.
  • Walking Shirtless Scene: Pretty much the male portion of Peek-A-Boo in nearly every scene they're in; Azusa also regularly walks around without a shirt, wearing just a bra to cover her top.
  • Weirdness Censor: Through the gang's alcohol-fueled binges and tendency to walk around naked, Nanaka's crush on Chisa is considered the weirdest thing about their group.

The First-Years

    Iori Kitahara
Voiced by: Yuuma Uchida

A freshman who has moved to Izu to attend its university's mechanical engineering program, Iori moves in with his uncle Toshio Kotegawa and cousins Nanaka and Chisa, only to get roped into Peek-a-Boo (despite his initial lack of swimming skills) which occupy the Kotegawas' family-run diving goods shop "Grand Blue".

  • Accidental Pervert: Many times towards Chisa. In Chapter 22 he caught her in various compromising positions during an all-girls' party, such as being dared by the girls to wear a sexy two-piece bikini and later being groped by Nanaka and Azusa. And in Chapter 42 he caught her topless while undressing after she fell over the beach in a drunken stupor.
  • Adaptation Dye-Job: In the manga, Iori's hair color is a very saturated teal; the anime changes this to a dull blue.
  • Attractive Bent-Gender: Of all the primary boys, Iori gets this treatment the most.
    • During the "Boycon" Beauty Contest at the end of Izu University's festival, Iori gets help from Azusa to pose as her in order to humiliate Captain Kudou, who gets played for a fool via an Unsettling Gender Reveal when he thought "Azusa" accepts his confession after Kouhei pulls an earlier one on "her"; Iori ultimately wins the pageant.
    • When forced to work at the maid cafe during the Oumi Festival (alongside Kouhei), Iori can pull off a maid outfit and ponytail surprisingly well, at least enough that his first-year engineering friends don't immediately recognize him. They even decide to flirt with him.
  • The Beard: To Chisa; following the Izu Spring Festival, he's forced to pretend as her boyfriend to make sure she doesn't get unwanted attention, lest he face the wrath of Nanaka for failing to do so. Most of the Izu University boys, especially in mechanical engineering, think the pair are a couple (thus making him a target of their jealousy), but Iori can't do a thing to refute it. He later becomes more tolerant of this, and uses it alongside Chisa as a method to prevent her from getting hit-on.
  • Big Brother Instinct:
    • A flashback of his childhood has Iori finding Shiori after she got lost in the woods. Present day, while their relationship soured a lot since, Iori makes it his priority to defend his little sister from perverts, especially Kouhei.
    • In Chapter 49, towards the end of his, Chisa and Kouhei's trip to his family-run ryokan (to claim Iori's passport from Shiori so he can join the upcoming Palau trip), realizing that Kouhei and two male German otaku try to help him by luring her out using a mobile lolicon hentai game as a playbook, he goes to great lengths to wrestle them away from Shiori, who gives it anyways partly as gratitude to him protecting her, partly to keep him away from Carina, the other perverted German otaku.
  • Bishōnen: While not as pronounced as Kouhei and Naomi, he's still one of the most handsome boys in the cast, with delicate enough facial features to make for a pretty convincing crossdresser.
  • Book Dumb: His notes at college are abysmally bad, and his parents consider him dumb as bricks (though it's implied that they're more upset at him refusing to inherit their ryokan). As he had the full intention of having a fulfilling college life, it's not quite known how Iori would operate if he wasn't forced by his senpai to binge-drink almost every night.
  • Butt-Monkey: Iori receives an awful lot of abuse from almost everyone he interacts with. As a matter of fact, Toki and Kotobuki consider both his antics and dirt-poor decision-making hilarious and highly entertaining.
  • Chick Magnet: He has some Ship Tease with Chisa (despite their more sibling-like dynamic and being cousins by adoption), Aina and Naomi obviously have a crush on him, and Azusa and Sakurako have at least expressed some interest in him.
  • Chivalrous Pervert: In spite of the raunchy stuff that he engages into, it's shown that Iori cares a great deal about the happiness of his friends, and whenever stuff would get so heated the girls might be affected, he is the first to put his foot on the ground.
  • Distracted by the Sexy:
    • Downplayed. During the Okinawa trip, Iori had to sleep elsewhere because the men's room is too small for him, Kouhei, Shinji and Ryuujirou. Sure enough, he had to spend the first night between Nanaka and Azusa, two of the sexiest girls in the series, which would be a good thing but for the fact that he, Kouhei and Aina also had to take up diving licensure exams the next day. He tries his best to keep himself focused, such as spending the night studying and choking himself unconscious, but his libido ultimately gets the better of him when he miswrites his name as "Boobs" while taking written exams in front of Nanaka, earning him a salvo of slaps across the face from an overlooking Chisa.
    • Invoked. When he lands in an uncomfortable situation with Nanaka which leads her to believe that Iori is attracted to her, Iori states that he does find her stunning, but that this is not the kind of relationship he wants with her, as he considers her among his closest family, almost a sister. This eases Nanaka's worries.
  • Disappointed in You: Being who he is and how he behaves, Iori rouses this out of plenty of people:
    • This is the default when it comes to Chisa, who through their interactions makes it more than clear that she expects an awful lot better from Iori than what little he shows her.
    • This happens aplenty when it comes to Aina, who knows him to be a better person than he shows, and has a crush on him due to having a firsthand experience of his better personality traits.
    • Played for Laughs when it comes to Tokita and Kotobuki, who place unrealistic standards on him, especially when it comes to binge-drinking and "club activities" (it doesn't help that Iori isn't as much of a Bunny-Ears Lawyer as the two).
    • His parents consider him a dumbass, though it's implied that they're more upset at him for refusing to inherit their ryokan.
  • Even the Guys Want Him: Not only can Iori rock being an Attractive Bent-Gender, but Naomi is head over heels for Iori, much to Sakurako's chagrin.
  • Everyone Has Standards:
    • Despite his mutual antagonism with his little sister Shiori, he still knows better than to leave her alone with Kouhei.
    • He tells Sakurako that even though he's not above criticizing and making fun of Kouhei for his otaku tendencies, he's never once considered depriving him of what makes him happy or forcing him to become someone he's not.
    • Iori refuses to tolerate messing with someone's fundamental dignity or happiness, so he's very given to exact justified retribution for those wrongs. As a matter of fact, this is the main reason girls like Aina and Sakurako are attracted to him (the former, for protecting her from her former clubmates; the latter, exactly for being criticized over her harsh reaction to Kouhei's hobbies).
    • Even beyond the club's standards (as shown in the club's entry of this trope), Iori does not tolerate anyone looking funny at his cousins Chisa and Nanaka, or at Aina and Azusa for that matter. This is not something wise to do with Iori present, as he'll find the most underhanded method available to him to get back at whatever idiot who dares to try.
  • Foil: To Chisa. Iori did not want to inherit his parents' Ryokan and fled his family home to live with the Kotegawas and enroll in college, while Chisa had to enroll in college to appease her mother, who disapproves of her dream of becoming a diving instructor, and thus maybe inheriting the family business. As such, Chisa and Iori find themselves sharing the same space and becoming close friends after they came from two completely opposite backgrounds, Iori having been inside and looking out, while Chisa is outside and looking in.
  • The Heart: Despite his rotten personality, Iori is a surprisingly good motivator. He makes Aina realize there's no such thing as a "perpetual laughingstock", while he calls out Sakurako's criticism of Kouhei's otaku lifestyle when there's more to him than just that and later comforts her after her Love Confession got rejected by Naomi.. On these occasions, his Peek-a-Boo seniors and engineering friends, respectively, follow his lead.
  • Heroic Comedic Sociopath: He's not shy about throwing his friends under the bus (so long as it's minor stuff). He's so unapologetically prone to playing dirty that more than once, Sakurako has told him that she absolutely loves that side of him.
  • Hidden Depths:
    • Iori has a fairly good hand-eye coordination, allowing him to easily flip Chisa's skirt with nothing but rubber balls and hold his own in a tennis match against the Tinker Bell boys.
    • Though it's overshadowed by his bad grades, poor decision making and reactionary nature, it's shown that Iori is a deeply kind individual, a reliable friend when things get serious, and a person whose enthusiasm is his greatest asset.
  • Hoist by His Own Petard: During the Okinawa trip (which also doubled as the freshmens' licensure exams), upon learning from Nanaka that he is risking failure due to his difficulty with goggle-clearing exercises, he tries his best to train in secret for fear of being teased by Kouhei and Aina, and gets Chisa (and later Aina) to help submerge him in the backyard pool of their lodging. However, one onlooker mistook the training for a crime scene, and when a cop shows up Iori cannot leave the pool after he goes skinny-dipping later that night, causing him to fall ill the next day and miss out on the practical exams which Kouhei and Aina pass.
  • Jerk with a Heart of Gold: On one hand, most of Iori's motivations aren't entirely for good intentions and can be obnoxious, selfish, unreasonable, and cruel to Kouhei, his engineering friends, and Peek-A-Boo members (and vice versa). That said, if a third-party decides to be a dick to his friends, he will go to great lengths to give them what's coming to them, as Kudou and Sakurako found out the hard way. In addition, despite his mutual antipathy with his younger sister Shiori, he will still act as a good brother to her when the situation calls for it.
  • Like Brother and Sister: Iori holds Chisa and Nanaka very close to his heart. With the latter, after clearing a few misunderstandings, he clarifies to Nanaka that he regards her as a surrogate Cool Big Sis; with the latter, their relationship are akin to twins who deeply understand each other enough to think alike, such as cooking in tandem and finishing each other's sentences.
  • Likes Older Women: By his own admission, Iori finds Azusa and Nanaka more attractive compared to his fellow freshmen Chisa and Aina.
  • Mistaken for Gay: Due to Iori's attempt to appeal to hardcore siscon Nanaka, Azusa thinks Iori is not only bisexual, but also in love with Kouhei. He doesn't have the heart to tell her she's wrong after realizing she, who openly identifies as bisexual, is excited over someone who apparently shares the same interests as her.
  • Mr. Fanservice: Besides being good-looking, among the Peek-a-Boo boys Iori (as well as Kouhei) is frequently seen naked.
  • My Greatest Failure: Albeit Iori usually shows little remorse about the outrageous stuff he engages into, the aspect he regrets the most is how hilariously badly he hit off with Chisa. The other stuff is no big deal, but Chisa's approval and friendship is paramount to him, even though he is remarkably bad at showing it.
  • Oh, Crap!: When trying to escape Chisa, who is livid at having been seen topless by him, Iori realizes that she stopped chasing and notices a lone shoe next to a ravine. He catches a glimpse of an unconscious person on the bottom, and thinking it's Chisa, he immediately careens down the ravine. He finds that the unconscious person is Kudou, who once regains consciousness, tells Iori that the person he was expecting must have been so dear to Iori that he saw no trouble whatsoever in throwing himself down a ravine for their sake.
  • Old Shame: In-Universe, Iori wrote several original songs back in middle school and is absolutely embarrassed by them. His sister Shiori not only found them, but also gave Chisa a CD recoding of those songs as performed by their father, in order to blackmail Iori into returning home for the summer break.
  • The Reveal: He's the son of a ryokan owner, who was originally meant to succeed the family business, but instead ran away to college, leaving his sister to inherit it. In addition, Toshio also casually reveals that he isn't even remotely blood-related to Chisa and Nanaka, his father being the adopted brother of their mother.
  • Saying Too Much: Iori has a nasty habit of speaking his mind too much, resulting in misunderstandings, primarily with Chisa.
  • Throw the Dog a Bone: After failing to earn his professional diver's license during the Okinawa trip when he went down with a fever on the day Kouhei and Aina took their exams, he finally gets it in Chapter 28, albeit barely and in a simpler, private session made for him on the waters just outside Grand Blue. Moreover, he earned it in the presence of Shiori, his younger sister and current (albeit reluctant) heiress to their family-run ryokan, who has been pressuring him to return home and resume the position neither of them truly wanted.

    Chisa Kotegawa
Voiced by: Chika Anzai

Iori's cousin and fellow mechanical engineering freshman at Izu University. Unlike her clubmates, she typically shies away from their "extracurricular activities", instead focusing on her studies and diving.

  • Academic Athlete: Not only is she a competent diver, she is also a fairly intelligent student, as shown when she scores an impressive 80s on a German test Iori, Kouhei and their friends flunked even with cheating. She is by far the most academically proficient among the underclassmen, though it shows that she lacks the life experience of their seniors.
  • A-Cup Angst: Downplayed—while she actually has decent breasts (unlike Aina), she sometimes becomes self-conscious whenever she is reminded that she's smaller compared to the positively bulging Nanaka and Azusa. Later in the story, while tending to an ill Aina after a night of hard partying, Chisa catches multiple glimpses at Aina's bookshelf, which is filled to the brim with breast-growth books and guides. She tries to sneak one out, but Aina catches her red-handed.
  • Adorkable: For all her serious attitude, she noticeably perks up whenever diving is involved, and her rather cute moments began increasing in frequency as she starts to open up.
  • Because I Said So: Played with. In Chapter 55, it's revealed that her mother not only disapproved of her dream of becoming a diving instructor, but forced her to take up engineering at Izu University, creating tension between them during the freshmen's Palau trip. Then it is revealed that her mother is merely worried that Chisa is also developing barotrauma-induced hearing problems on her left ear, much like herself.
  • Beware the Quiet Ones: As Iori and Kouhei fully know, eliciting Chisa's wrath is not a wise thing to do, even though they do it all the freakin' time.
  • Boyfriend Bluff: After winning the Miss Izu contest, he declares Iori her "boyfriend" in front of the university's student body, partly to protect herself from unwanted suitors, partly to spite Iori for making her win in a most embarrassing manner. Iori later agrees with the charade, as Chisa is very prone to getting hit-on.
  • Broken Pedestal: Invoked and played with. It's shown that Chisa was expecting great things when Iori moved in, as she is fond of her memories with him during the time they spent as children. She is sorely disappointed when she realizes how easily he's goaded into becoming a party animal by the senpai of the diving club. It's also shown that in spite of this, she's expectant of seeing Iori getting his shit together, and has been fortunate of being able to catch glimpses of his better side.
  • Chaste Heroine: Chisa seems to have no interest in dating at all and is more interested in finding diving friends.
  • The Comically Serious: She typically responds to most of her clubmates' (especially Iori's) shenanigans with no worse than an expression of exasperation.
  • Dandere: Chisa is occasionally shy, but will speak her mind when angry or goaded enough via Death Glare and/or Tranquil Fury.
  • Death Glare: One of her more common expressions due to having to put up with her clubmates' alcohol-fueled shenanigans on a near-daily basis.
  • Dull Eyes of Unhappiness: Played for Laughs near the end of chapter 35, after Naomi confesses to her that he likes Iori.
  • Entertainingly Wrong: She has always been under the impression that her mother doesn't think too highly of her goal of becoming a diving instructor, mistaking her worries for callousness. It turns out that her mom is worried that Chisa might lose her hearing on her left ear from barotrauma like she did.
  • Hidden Depths:
    • It's shown that deep inside, she wants to have a friendlier relationship with Iori, but he's not making it easier for either her, or for himself.
    • Chisa, under normal circumstances, is markedly kind, sweet and fun-loving, and has the virtue of a childlike curiosity for both her hobbies and sharing them with her friends.
  • I Just Want to Have Friends: She has always wanted to have diving friends of her age, which is why she was so expectant to live with Iori... that is, before Toki and Kotobuki dragged him from his feet, stripped him naked and wet his whistle almost nightly. While she's not that appreciative of the nightly partying of the club, she's deeply grateful of having Iori, Kouhei and Aina for being there as friends she can dive with.
  • Just Friends: This forms a great part of her relationship with Iori, as he is The Beard for her, and that sort of relationship doesn't work if the two parties are not friends to begin with. Though she is greatly dismissive of Iori every time he wants to involve her into something fishy, it's shown that Chisa does care a great deal about him, and is, for lack of a better term, one of the best friends she's ever had.
  • Kissing Cousins: Invoked and subverted. She is Iori's girlfriend only at college, in a mutual effort by both Iori and Chisa to prevent her from being hit-on. Neither have shown the actual interest to pursue a relationship of that nature, as they consider each other as their immediate family and by no means significant others, regardless of the fact that they're not blood-related.
  • Like Father, Unlike Son: Zigzagged. On one hand, Chisa inherited her quiet demeanor from her mother, and takes an awful lot after her; on the other hand, they rarely see eye-to-eye because her mother disapproves of her dream of becoming a diving instructor; then it is revealed her mother is merely worried she is also going deaf on her left ear, much like herself.
  • Locked Out of the Loop: While the rest of Peek-a-Boo knows about Nanaka's abnormal love for her, no one is willing to tell Chisa about it for fear of how Nanaka will retaliate.
  • Morton's Fork: Played for Laughs when Chisa asks Iori about a question on breasts; he deduces no matter what answer he gives, it's going to be the wrong one regardless.
  • Nice Girl: Played with. She's one whenever her friends approach her as civilized people... that means, almost never. On those rare occasions, she's shown to be a kind, gentle and doting girl.
  • No Social Skills: As the series goes on, it becomes clear that she has little in the way of socialization, often focusing on diving. It is through interacting with fellow freshmen Iori, Kouhei and Aina that she starts opening up.
  • The Not-Love Interest: Played with. Aina considers her as serious competition for Iori's affections, even though he has stated that he doesn't have those intentions towards either Chisa or Nanaka. However, it has never been shown what Chisa's expectations of Iori are, as she seldom thinks about that sort of thing.
  • Not So Above It All: Most of the time Chisa is an introvert with a frugal approach to drinking alcohol. When she does get hammered, though, she can get quite a bit dorky, and during their first night at Palau she has taken to drinking together with Iori and Kouhei, and is found sleeping on top of a dog and next to the half-naked boys by Aina the following morning.
  • Oblivious to Love:
    • Chisa has no idea of Nanaka's unsisterly love for her. Worse yet chapter 49 implies that because of this, she is totally used to being glanced at lustfully and doesn't see it as bad.
    • Until fairly recently, it's shown that Chisa has been largely unaware that Aina likes Iori. As such, her mother has to point out that Aina has been grooming up in an attempt to win him over, which didn't quite sit well with her.
  • Only Sane Man:
    • In a cast full of alcoholic, nudist weirdos, she stands out for her reluctance to get roped into their shenanigans and trying to bring order out of the chaos they create, so much so even her quirky obsession with diving would look normal.
    • This also applies to her classmates at college, which are a bunch of hormone-addled jerkasses.
  • The Quiet One: Chisa is PaB's resident wallflower. It's shown that Chisa is not one to speak much or draw attention to herself.
  • Reluctant Fanservice Girl: Chisa has her skirt flipped by Iori during the Miss Izu Beauty Contest (in order for her to win the contest and thus earn the club money), has been forced by Nanaka to wear a dress and skirt for the club's tennis match against Tinker Bell, was forced to join Azusa and an equally-reluctant Aina in a part-time job handing out flyers dressed in skimpy outfits to earn back the money she and her clubmates spent on shopping diving gear, was caught by Iori being groped by Nanaka and Azusa, and was again caught topless by Iori while undressing after she got drenched after falling onto the beach in a drunken stupor.
  • School Idol:
    • After winning the Beauty Contest (albeit after some skirt-flipping antics by Iori), Chisa has become the toast of Izu University, specifically the mechanical engineering program (if only due to the sheer dearth of girls taking up the program, the male-to-female ratio being 140:3).
    • One of the graces of the beauty pageant ordeal comes from the fact that her clubmates consider her so pretty that they know for a fact that she'll win it by just participating. This praise alone makes Chisa willing to participate.
  • Shared Family Quirks: When the freshmen visit Palau, they note that Chisa has taken a lot from her mother, from her dark hair to her introverted personality, as well as a tendency to be a bit of a dork in private, but also hearing problems on their left ear.
  • Shrinking Violet: While not as crippling as Aina's case, it's still obvious that Chisa is introverted and socially awkward, as shown when she gets flustered over being complemented "nice buddy" by her clubmates during her tennis doubles match with Aina, which she understandably mishears as "nice body" given that she was wearing a skimpy shirt-and-skirt combo (specifically prepared by Nanaka for her), and shies away from Iori and Kouhei partying with their college friends in the former's room.
  • Sugar-and-Ice Personality: Despite her serious attitude and a tendency to occasionally react negatively to the shenanigans around her, Iori compliments her that she is far kinder than she appears, such as foregoing the chance to join Kouhei and Aina in deeper waters during the Okinawa trip to join Iori back on the boat, thinking he may be sulking over being left behind after failing to take the practical exam (and thus earn a diver's license that allows him to go to open waters) due to going down with a fever yesterday; he was actually merely seasick, but he nevertheless appreciates her concern.
  • Tears of Joy: [[In Chapter 57, she tearfully thanks Iori for letting her know why her mother is so against her diving instructor goals (which is a more health-related matter than she thinks) and tells him that she's glad to have him close to her.]]
  • The Three Faces of Eve: The Child to Aina's Wife and Azusa's Seductress, being the shortest of the three Peek-a-Boo girls and the most socially-awkward, with a childlike fascination for the sea.
  • Tranquil Fury: Aside from the use of Death Glare, Chisa has problems expressing her anger and retaliates using this trope instead, be it the girls from the Tinker Bell tennis club in a tennis match or Iori Saying Too Much.
  • When She Smiles: It's a rarity for her to crack a smile, given her serious attitude and having to put up with all the alcohol-fueled shenanigans around her on a daily basis, but when she does, she's downright adorable (such as when Iori expresses his gratitude for her showing him the wonders of scuba diving).
  • You Keep Telling Yourself That: In spite of her low tolerance of how Iori behaves, it's shown that Chisa cares a great deal about him, and firmly expects better things of him rather than what little he has shown her since they've been living together. It's apparent how much she wants to respect him, but she can't. However, Chisa does see from time to time the better side of Iori, just like she wants him to be.

    Kouhei Imamura
Voiced by: Ryohei Kimura (main) / Shouta Aoi, Inori Minase and Rie Kugimiya ("mix voices", Episode 8)

A hardcore otaku and a fellow mechanical engineering freshman at Izu University alongside Iori and Chisa, the former with whom he has struck a relationship constantly zigzagging between friendship and enmity.

  • Berserk Button: Being the otaku that he is, he hates it if someone in real-life is experiencing typical anime and manga clichés, such as a childhood friend calling someone "onii-chan", and will turn violent from jealousy.
  • Bishōnen: Most of his female schoolmates admire his good looks (only to be rebuffed by his hobbies), which in turn makes him the envy of his male schoolmates.
  • Celibate Hero: Kouhei states he's only interested in 2-D and has turned away real girls who are interested in him for that reason.
  • Comedic Lolicon: It's mixed with Pædo Hunt. Kouhei has admitted to being interested in 2-D kids, but while he's mostly harmless, the rest of the cast react with fear and horror when his interests appear to be about real children. This is also the reason he later becomes attracted to fifteen-year-old Shiori, much to Iori's alarm (despite their mutual animosity), while he states that Chisa looks too much of a teenager to fit his perfect "little sister" mold.
  • Covert Pervert: Overlapping with Not So Stoic. While Kouhei tries to act like he's not interested in "3-D women", he will jump at the chance to see Azusa or Nanaka naked, to say nothing of his infatuation with Kaya and later obsession with Shiori.
  • Crazy Jealous Guy: When Kouhei thinks someone is interested in Shiori, his first thought is murder.
  • Green-Eyed Monster: While Kouhei doesn't care to get a girlfriend, he hates the idea of others having girlfriends.
  • Harem Seeker: Kouhei originally wanted to join a club filled with beautiful women, hoping to be the only man to gain a harem.
  • Irony: The funny thing about Kouhei is that he's handsome enough to have a harem, but his own weird expectations and standards, and lack of self-awareness keep him from even recognizing the multiple hints that would lead him to having one.
  • Little Sister Heroine: A major fan of this character archetype, Kouhei gets very jealous over the idea that Shiori might have feelings for her older brother Iori or vice versa.
  • Nice Job Breaking It, Hero!: His and Iori's plan to force Chisa to set them up a mixer for their engineering friends through a rigged King's Game backfires when, holding the "King" stick in the last round, instead of picking whoever has the "1" or "3" sticks to set up a mixer (both numbers which Iori predicts may belong to Chisa) he misreads Iori's sign language and declares "4". Not only does this derail their plan, but it puts them at the mercy of Shinji... who wastes no time introducing Iori and Kouhei's friends to his fellow beefcakes from the local rugby club.
  • Otaku: Kouhei openly shows his love for anime and manga, considering his default attire is a shirt with his favorite character "Rarako-tan". However, he seemed to believe no one could really perceive him as one until he explained himself; partially true whenever a woman doesn't recognize the apparel he wears.
  • Proud to Be a Geek: Is absolutely shameless about his Otaku interests and worship of his favorite Idol Singer Kaya Mizuki.
  • Shipper on Deck: By chapter 51 even he can see that Aina is head over heels for Iori and does his best to help her out ... by ruining Iori's chances with any other girl. It's implied that he's not doing it to spite Iori (as Iori percieves), but rather, for Aina's sake.
  • Voice Changeling: Kouhei is capable of altering his voice, shown during the engineering gang's Coitus Interruptus antics against Yuu and Rie in Episode 8, pretending to be three different girls Yuu was simultaneously dating besides Rie.

    Aina Yoshiwara
Voiced by: Kana Asumi

A freshman at Oumi Women's University, Aina, or "Cakey" as Iori and Kouhei call her, is part of the Tinker Bell tennis club, but leaves to join Peek-A-Boo.

  • A-Cup Angst: Aina has the flattest chest among the Peek-a-Boo girls, and often feels inadequate compared to Nanaka and Azusa. After coming back from her hometown during the summer however, they have suspiciously, and surreptitiously, increased in size.
  • Aborted Declaration of Love: Aina lets her budding affections out in her native dialect to Iori during their date, but immediately takes it back, pretending she was talking about olives, when she says "I love you".
  • Adorkable: Outside her makeup, Aina is cute and painfully shy.
  • Beautiful All Along: Aina is a natural cutie; she just needs to lay off the excessive makeup she puts on whenever she goes out in public.
  • Because You Were Nice to Me: She joins Peek-a-Boo as an expression of gratitude to Iori and Kouhei for defending her honor and standing up to her former clubmates at Tinker Bell for humiliating her.
  • Beneath the Mask: Whenever Aina has make-up on, she's bold and fearless, but without it, she's an Adorkable Shrinking Violet.
  • Can't Hold Her Liquor: Only in terms of the series, but when she drink like the rest, she's one mean drunk; Justified when the club drinks strong alcoholic content (even diluted was "only" 30% when typically it's higher than 90%), but her drunken behavior often borders psychotic with regards to Iori's obliviousness to her affections for him. And without Iori around? She's just like the seniors.
  • Clingy Jealous Girl: She usually dislikes Iori hanging out with women, usually with Chisa and Busujima.
  • Corrupt the Cutie: Played for Laughs with her experience as a Peek-a-Boo member, having learned, in only a space of a few days, to participate in the boys' drunken binges and stripping down to her underwear.
  • Cosmetic Catastrophe: She has a really bad taste in makeup, often applying excessive amounts of foundation, blush, lipstick, eye shadow, and eyelashes.
  • Covert Pervert: While Aina usually tells off Iori and Kouhei for stripping on inertia, she actually does like looking at their bodies.
  • Embarrassing Nickname: "Cakey", a derogatory reference to her heavy, horrid appearance upon using so much make-up.
  • Everyone Can See It: Played with. Aina's crush on Iori is so transparent every Senpai in Peek-A-Boo knows how big it is, yet her same-age friends are not the introspective kind, so they are none the wiser. When she back-tracks her accidental Love Confession to him during their date, her slip-up is within earshot and totally obvious to the patrons at the restaurant.
  • Fanservice Pack: Aina started out with the smallest bust size among the women in the club but after returning to the club following hard labor at her hometown, her breasts have increased in size tremendously. Neither Iori or Kohei want to ask what she did to increase them and even Chisa wanted to know; She's padding them, and badly at it.
  • Heroic Self-Deprecation: Aina has issues with her body image, especially due to the fact that she has to share the limelight with Chisa, Asuza and Nanaka, who are gorgeous and way more buxom than her. This makes her think that they're competition for her in her pursuit of Iori, and as she percieves herself, she thinks that she's on the losing side compared to them.
  • Hidden Agenda Villain: Played for laughs and subverted. During the mixer she sets up for Iori and his college friends, she does everything she can to sabotage any possibility Iori has to form a relationship with any of her friends both by caking them up with makeup and by intentionally telling them beforehand that Iori has a girlfriend. Playing it by ear and relying on Iori's friends' own thirst for girls, Aina successfully puts a lid on the mixer. As such, Iori considers Aina the villain in his book, not knowing that she did it because she's precisely aiming for him, so her intentions are more pure than he realizes.
  • I Just Want to Be Loved: A shy, unassumingly cute girl, herself, versus a trio of bombshells, Asuza, Nanaka and Chisa, aiming to focus Iori's attentions on herself instead of them. Ironically, Aina has the best chance out of the four just due to the fact that she's actually trying. It's shown to be an uphill battle for her, but she's putting her all on it.
  • I Just Want to Have Friends: After being forced to enter the "joke category" at the Miss Izu Pageant (alongside Chisa), a drunken Aina admits she just wanted to fit in with her college peers, even if it meant making a fool of herself.
  • Nice Girl: Among the club members, Aina is the most openly pure-hearted and good-natured.
  • Only Sane Man: Upon joining Peek-a-Boo, Aina becomes this for the club, but it doesn't last long if she starts drinking herself.
  • Open Secret: Played with. Neither Kouhei nor Chisa were aware of her attraction to Iori until Chisa's mom spelled it out for them; however, upon closer observation, Kouhei realizes that Aina is completely in love with Iori, way beyond mere infatuation.
  • Pet the Dog: The reason she has a crush on Iori, as he helped her during her most abject time and showed her that there are people who will accept her unconditionally.
  • Put on a Bus: She returns home for the summer to help her family out in their farm; Played for Laughs when Azusa makes it sound like she isn't coming back at all. Aina returns to the story in Chapter 50.
  • She Cleans Up Nicely: Inverted—Aina's make-up skills kill any natural beauty due to how garishly cakey and heavy it is.
  • Shrinking Violet: Without her makeup Aina is terminally shy and unable to interact with almost anyone outside Peek-a-Boo.
  • Small Town Boredom: Aina is from a farm family and she seems pretty embarrassed by it.
  • So Proud of You: She's deeply touched when she learns of the great lengths that Iori and Kouhei were willing to go through to prevent her from being forced to leave due to her family refusing to pay for her tuition, only to learn that it was only for the summer break.
  • Tears of Blood: Aina cries these when offering Shori her swimsuit in fear that her clothes will be the same as a middle-schooler's.
  • The Three Faces of Eve: The Wife to Chisa's Child and Azusa's Seductress, being the most openly kind-hearted of the three Peek-a-Boo girls as well as the one with the most obvious crush on Iori.
  • Trash of the Titans: As Iori, Chisa and Kouhei discover on their first night at Palau, Aina isn't exactly a great dishwasher, seen with the pile of unwashed kitchenware on the sink of their lodging.
  • Unknown Rival: Played with. Aina is under the impression that she has to compete with Chisa, Azusa and Nanaka for Iori's affections; Iori himself is not shy in expressing an admiration for their beauty, but he considers Chisa and Nanaka his closest family, while he doesn't pursue Azusa out of respect for the club's order, meaning that Aina has a better opportunity with Iori than she thinks.
  • What the Hell, Hero?: During one of their first dives together, Iori chides her for lying about how much air she had left in her tank. The senpai do not chide her, however, but tell her that she should keep in mind that she should rely on them because they're there for her.

The Upperclassmen

    Ryuujirou Kotobuki

One of the headlining third-year members in Peek-A-Boo and studying for his mechanical engineering undergraduate.

  • Big Brother Mentor: A surrogate version to his underclassman Iori in the earlier stages of the series, not only teaching him the basic of scuba diving, but also giving him words of wisdom.
  • Chivalrous Pervert: He is an excellent listener and he's very popular with the ladies at his bartending job for his willingness to lend a shoulder to cry on. That is, notwithstanding that he's a binge-drinking nudist.
  • Cloud Cuckoo Lander: Alongside Toki, they have extremely off-the-wall logic regarding what's decent, what's acceptable and what's healthy.
  • Early Installment Character-Design Difference: Ryuujirou was much slimmer, but still muscular, in the first chapter; subsequent reappearances render him just as bodily wide as Shinji.
  • He Cleans Up Nicely: While working as a part-time bartender at a local bar popular with college students, Ryuujirou dresses up for the part, complete with mannerisms and an air of grace completely unlike what he's usually like in Peek-A-Boo.

    Shinji Tokita
Voiced by: Hiroki Yasumoto

Another third-year member of Peek-A-Boo also in the mechanical engineering program.

  • The Big Guy: Of the male club members he is the most musclebound (even more so than fellow senior Ryuujirou), so much so he can easily take up home-moving jobs on consecutive days (in contrast to Iori and Kouhei, who could barely carry furniture together).
  • Hidden Depths: He's the only person in Peek-A-Boo to be in a relationship.
  • The One That Got Away: It's shown that Azusa has a thing for him, but she does not act on it because of Toki's girlfriend.
  • What Does She See in Him?: At the very least Iori and Kouhei were shocked that he had a girlfriend to the point of wondering if she was even human.

    Azusa Hamaoka
Voiced by: Toa Yukinari

A third-year student from Oumi Women's University.

  • Adaptational Ugliness: Downplayed, but a common lament of manga readers is that the anime shortened and simplified her "sex hair" quite a bit, to make it easier to animate.
  • Bedmate Reveal: How she makes her debut in the series—she casually lies next to Iori to sleep there during the night, without waking him up; he finds her next to him in the morning not knowing who she is.
  • Bi the Way: On one hand, she is explicit about her desire to sexually violate Nanaka to Iori; on the other hand, it's implied she also has a thing for Shinji (but couldn't get reciprocated given he already has a girlfriend), and during the Okinawa trip she also proposes to sleep with Iori after sensing that he has some feelings for her.
  • Cloudcuckoolander: Azusa loves being part of the chaos surrounding Peek-A-Boo, even undressing along with the men in their drunken escapades.
  • Cool Big Sis: When she's not being a drunken mess, she would occasionally give sound advice to her underclassmen.
  • The Gadfly: She has said and done things just to mess with others for fun.
  • Gag Boobs: Azusa has the largest breasts among the three Peek-a-Boo girls, and is prone to some Gainaxing, so much so even Chisa—herself no slouch in terms of bust size—cannot help but feel insecure after she was barely able to fit her vest on Azusa when the latter walked into their Izu Spring Festival booth near-completely topless.
  • Hard-Drinking Party Girl: While everyone in Peek-a-Boo drinks alcoholic beverages, among the girls she is the most eager to join her male clubmates' binges.
  • Has a Type: As she states, when it comes to boys, Toki; when it comes to girls, Nanaka.
  • Lovable Sex Maniac: Azusa isn't shy about flirting with everyone, having sex talks or speak of her obsession with wanting to bed Nanaka, including giving Iori a chance to sleep with her once.
  • Metaphorically True: How Azusa explains to Iori about Aina's situation when she has to return home—she deliberately withholds the fact that Aina merely has to help her family at their farm during summer break, instead claiming she needs to leave because she's out of college expenses.
  • Ms. Fanservice: Nanaka and her are the go-to girls when it comes to fanservice.
  • Nice Girl: Ostensibly the nicest of the senior club members and the most reliable, even with her own brand of weirdness.
  • One of the Boys: Among the few girls in Peek-a-Boo, she is the most eager to join the boys' activities.
  • Please Put Some Clothes On: How Chisa reacts to her walking into their Izu Spring Festival booth wearing nothing on her top but a bra.
  • Secret Test of Character: She subjects Iori to one when she proposes to him to have sex. Gauging his response, Azusa determines how far he'd go to either protect or mess up their friendship, even when she was entirely willing to go through it either way.
  • Shameless Fanservice Girl: Not only is Azusa the bustiest and curviest among the Peek-a-Boo girls, but she also has little qualms stripping down to her underwear and join the boys' drunken escapades. And that's not to mention how she was introduced, sleeping half-naked next to four completely naked men.
  • Shipper on Deck: She makes Iori keep up the false Relationship Upgrade with Chisa, knowing there's small sparks between them, while giving advice to Aina before the latter goes on a date with Iori. Azusa even believes an Iori/Kouhei pairing is possible.
  • Spanner in the Works: Iori and Kouhei's attempts at getting Chisa drunk in order to force her to enter the Miss Izu Contest backfire spectacularly when not only does Azusa unwittingly prevents them from doing so, but also ultimately catches up to what they are doing and forces them to confess.
  • Supreme Chef: She has the best culinary skills among the club members, which is why she was invited by Ryuujirou and Shinji to teach the freshmen how to cook okonomiyaki for the Izu Spring Festival.
  • The Tease: Azusa knows how to tickle her clubmates' sexual urges, and isn't shy about flirting with everyone.
  • The Three Faces of Eve: The Seductress to Chisa's Child and Aina's wife, being the curviest and most flirtatious of the three Peek-a-Boo girls.
  • You Gotta Have Blue Hair: Purple in her case.

Grand Blue Staff Members

    Nanaka Kotegawa
Voiced by: Maaya Uchida

Chisa's older sister, professional diving instructor, and manager of Grand Blue.

  • Ambiguously Gay: While her sexuality is never flat-out stated, her affections towards her sister are, at least to some degree, that of a sexual nature.
  • Beware the Nice Ones: Nanaka is usually the most mellow character in the main cast, but as Iori comes to discover (much to his horror), anything Chisa-related is bound to set off a dangerously psychotic side.
  • Camera Fiend: Whenever Chisa is going to be cute, expect to see Nanaka have a camera with an incredibly long zoom lens to take pictures.
  • Can't Hold Her Liquor: When Aina throws an all-girls' drinking party, Nanaka looks like she's hammered throughout the event; subverted when Toshio explicitly tells Iori the day after that she has high alcohol tolerance, a bad thing for Iori because he lets slip that he's in a false relationship with Chisa, which Nanaka overheard during the party.
  • Chaste Hero: Extreme love for Chisa aside, Nanaka doesn't get romantic love at all. She's generally puzzled when anyone shows any form of romantic or sexual attraction. This is pointed out by Toki and Kotobuki, and later Azusa implies that between Nanaka, Toki, Kotubuki and herself, their group dynamic functions the way it does because something did happen in the past.
  • Cool Big Sis: Besides being Chisa's actual older sister, she also acts as a surrogate one for Iori.
  • Gag Boobs: Nanaka has the largest breasts of all the main females (surpassing Azusa), and is prone to occasional Gainaxing. During the Okinawa trip, Iori can barely concentrate on his diving licensure exams after having to sleep between her and Azusa the night before, and even miswrites his given name as "Boobs" while taking a written test in her presence.
  • Entertainingly Wrong: Nanaka is left under the impression that Iori is a massive pervert and a porn addict. This is Toki, Ryuuji an Kouhei's fault, and due to awful timing on her part. Though he clears up most of the misunderstandings, Nanaka is still low-key wary of him.
  • Forbidden Fruit: As stated in Like Brother and Sister, Iori tells her that he won't deny her beauty, but that this attraction is not something he will act upon because he respects her and considers her family.
  • Innocent Fanservice Girl: Nanaka is pretty ok with stripping due to her job as a diving instructor.
  • Like Brother and Sister: Invoked. Iori tells Nanaka that he does find her to be stunning, but after he stumbles to make his point clear and makes things awkward as hell between them, he states that this is not what he wants from her, as he considers her his sister.
  • Lipstick Lesbian: Is shown to be very feminine, and currently shows no kind of attraction towards men. She is, however, shown to be quite fond of her own sister.
  • Nice Girl: There's little to nothing she's not willing to help with if she's asked to, that is, as long as it doesn't involve courting Chisa.
  • Noodle Incident: Toki and Kotobuki are quick to tell Iori that approaching Nanaka is a lost cause, implying that they were interested in her in the past and did try themselves to approach her to no avail. Later, Azusa tells Iori that a lot happened in the past between Toki, Kotobuki, Nanaka and herself, to explain why they are such close friends now.
  • Ms. Fanservice: Nanaka is the bustiest member of the main female cast, and is often shown in attire and/or postures that show off her generous proportions.
  • Paralyzing Fear of Sexuality: Nanaka becomes a blushing mess when topic of sex comes up and was terrified of Iori when she overheard him say he was attracted to her. He does clear up that this is not his wish, as he considers her like his sister.
  • Rage Breaking Point: She hits her when she finds Iori and Chisa on top of each other in Chisa's room in chapter 26.
  • Room Full of Crazy: Given her character archetype, Nanaka's bedroom is filled with pictures of Chisa.
  • Shared Family Quirks: Other than her yandere tendencies and raging lust for Chisa, by large Nanaka has inherited her father Toshio's mellow personality.
  • Yandere: Nanaka will kill anyone who attempts to flirt with Chisa, even demanding Iori ensures no man throughout the Oumi Women's University festival will hit on her; taken further when she catches Iori and Chisa in a compromising position, citing Iori must marry her sister.

    Toshio Kotegawa
Voiced by: Shinji Kawada

Chisa and Nanaka's father and owner of Grand Blue.

  • Ambiguously Brown: He has dark skin compared to his daughters, who don't share the same skin tone. Justified because he spends more time in the sun than the rest of the cast.
    • This trait is also shared by his wife as revealed in chapter 50 when Iori and co. went to Palau.
  • Satellite Character: Aside from being a relative of Iori, there is nothing much about Toshio as he rarely appears and doesn't join PaB in their parties or trips.
  • Wham Line: When Toshio is told about the possibility of Iori's affections going out of control for his cousins, he reveals his daughters and Iori are not related by blood.
    "That's crazy, even if they aren't blood-related."

Izu University

    Hajime Nojima, Shinichirou Yamamoto, Yuu Mitarai and Kenta Fujiwara
Left to right: Hajime, Yuu, Shinichirou and Kenta
Voiced by: Takuya Eguchi (Hajime), Jun'ya Enoki (Shinichirou), Natsuki Hanae (Yuu) and Robert Waterman (Kenta)

A group of first-year students in the mechanical engineering program, they strike up a friendship with Iori and Kouhei.

  • Childhood Friend Romance: Yuu is in one with Rie.
  • The Generic Guy: It's a Running Gag that Kenta is so physically bland and the most taciturn out of the group, no one pays any attention to him. This is particularly prominent during their visit to the Oumi Festival's maid cafe, when none of the waitresses take his order.
  • A Man Is Not a Virgin: These four embody this the most out of all the men at Izu University, though Shinichirou is the particular example among them. Taken to its logical conclusion where Iori proposes they help Shinichirou lose his virginity in chapter 36.5.
  • Mundane Solution: When Chisa is given a group ticket for four at the Oumi Festival, the men, including Iori and Kouhei, fail to impress on her why they should be invited. However, Kenta offers the best suggestion—if invited, he will not cause any trouble for her. The men are surprised because Kenta's solution of "what he wouldn't do to her" compared to their "what they would do for her" almost entices Chisa enough to say yes. Subverted when Rie gives Yuu a group ticket after some pleading, making their efforts moot.
  • Virginity Makes You Stupid: Zigzagged; with a goal of losing their virginity on their minds constantly, the four can be often seen as Book Dumb, yet if push comes to shove, such as a sudden pop quiz by the Sadist Teacher, they'll turn competent and logical, scoring in the acceptable low 70s.
  • Vitriolic Best Buds: They, Iori and Kouhei have such a bad habit of pissing off and/or trolling each other, any outsider would be forgiven for wondering why they are even friends in the first place; that said, if any outsider decides to be a dick to them, they will stand up in each other's defense, such as Iori criticizing Sakurako's own criticism of Kouhei's otaku hobbies, or standing up to Ushironomiya's sadistic teaching regimen.

    Captain Kudou
Voiced by: Jun Fukuyama

Captain of the Tinker Bell intercollegiate tennis club.

  • Butt-Monkey: His tendencies to be a jackass is offset by the frequency of his misfortunes, such as being humiliated in front of his own fangirls at the Izu Boys' Pageant after mistakenly confessing to Iori disguised as Azusa, feeling the wrath of Ryuujirou and Shinji's brute-force tennis tactics, and getting stuck on the bottom of a cliff with Iori who also fell while escaping Chisa, whom he caught topless earlier.
  • Handsome Lech: Kudou is one of the better-looking males in the cast, and in his first appearance alone he tries to hit on Azusa and Chisa, much to their chagrin.
  • Hoist by His Own Petard: His Handsome Lech tendencies come back to bite him in the ass when, during the Izu Boys' Pageant, he apparently out-charms Kouhei in proposing to Azusa... only for the "Azusa" among the audience to reveal "herself" as Iori in drag all along.
  • Humiliation Conga: In retaliation for his humiliation of Aina, and to prove a point to her that nobody will forever laugh at someone or be forever a laughingstock, Iori, Kouhei and Azusa set him up for a self-inflicted publicity disaster (see above).
  • Ironic Echo: According to Aina, he told her to go home now that he and their male clubmates were done laughing at her horrid appearance. Those words get thrown back at him by Iori and Kouhei after making a complete fool of himself by confessing to the former disguised as Azusa.
  • Jerkass: He's a complete asshole to anyone he wants to toy with, such as exploiting poor Aina's need for friends by making her enter the Miss Izu Pageant with heavy makeup.
  • Manipulative Bastard: During the Beach Episode, he tries to get Iori and Kouhei to let them hang out together with Peek-a-Boo in order to score with Chisa, Azusa and Nanaka. Seeing that the latter two aren't in any mood to negotiate, he challenges them to a series of tennis games. They agree, on the condition they don't hang out if they lose.
  • Smug Snake: Kudou is confident in his charisma and talents, which naturally makes him prone to a lot of rude awakenings once it turns out other people can outmatch him.
  • Took a Level in Kindness: By his third appearance, Kudou, while still having his own agenda, is a much civil person. Despite his rivalry with Iori and Kohei, he is actually on good terms with Ryuujirou and Toki. He also gives some advice to Iori on preparing a farewell party for the seniors.


An assistant professor in the mechanical engineering program.

  • Insistent Terminology: Despite still being an assistant professor and awaiting Izu University's approval, he insists he's a "full professor".
  • Large Ham: The man cannot say a single sentence without making an exaggerated pose.
  • Laser-Guided Karma: He gets his just desserts when Iori and company trick him into crushing his genitals after he forced them to do the same.
  • Sadist Teacher: He forced his students to crush their genitals via the Charpy Impact Test as revenge.

Oumi Women's University

    Kanako Iida, Kiyoko Kamio and Keiko Suzuki
Top to bottom: Kiyoko, Kanako, Keiko
Voiced by: Yuuko Gibu (Kanako), Yuka Iguchi (Kiyoko) and Ai Kayano (Keiko)

Aina's freshmen friends

  • Added Alliterative Appeal: Each of their given names start consonant-wise with a "k", and also end syllable-wise with "-ko".
  • Beautiful All Along: Much like Aina, the three girls are far better-looking without all that "cakey" makeup.
  • Berserk Button: They get pissed off when Aina reveals she has trouble putting on weight; Played for Laughs considering the three have far more shapely figures compared to Aina.
  • The Corrupter: Played for Laughs—Keiko admits whenever someone is innocent and pure, she feels the need to corrupt them. Lampshaded upon telling Kanako about a Pun on anal sex.
  • The Gadfly: The three love giving Aina a hard time about what to do with her feelings for Iori when she asks him out on a movie date.
  • The Reveal: Kanako is actually the younger sister of Idol Singer Kaya Mizuki, née Maya Iida.

    Rie Oohashi
Voiced by: Saki Yamakita

Yuu's Childhood Friend and freshman

  • Childhood Friend Romance: With Yuu.
  • Clingy Jealous Girl: Poor Yuu usually ends up a bloody pulp if Rie hears any hint of him flirting with other women.
  • Cute Bruiser: She's one of the strongest characters in the cast, comparable to Kotobuki and Tokita.
  • Muscles Are Meaningless: She's as strong as Kotobuki and Tokita, who are huge walls of muscle, while Rie is very slender.
  • Yandere: Zigzagged; on one hand, she thinks Yuu is cheating on her with other women when his friends are deliberately sabotaging him from having sex with her and Iori is Trolling him during the maid cafe event, making her physical repercussions on him more akin to that of a Tsundere, but when Yuu is caught "cheating" (he claims he merely exchanged phone numbers with a co-worker) and proposes to marry Rie in order to avoid her wrath, her Slasher Smile indicates she wants to monopolize him for herself.

    Sakurako Busujima

A freshman and Iori's bane

  • Bad Boss: She takes advantage of being Iori's senior at their part-time job to abuse him.
  • Black Comedy Rape: Invoked; Iori thinks her idea of "using her weapons" against Naomi is tricking him into a sense of vulnerability, tasering him unconscious behind his back, then dragging him to a Love Hotel to get what she wants from him.
  • Cry into Chest: A variant, after Sakurako's Love Confession got rejected by Naomi, Iori gives her a can of beer and comforts her. She then asks Iori to turn around for her to cry unto his back, stating that she doesn't want to cry unto his chest.
  • I Just Want to Be Loved: Implied. Beneath her claims of superficial relationship goals, her dedication to pursuing Naomi made even her friends question whether her boasts are truth. As Iori realizes, they're not.
  • Love at First Punch: She begins to harbor a crush on Iori for his guts to stand up to her and defend Kouhei's life choices.
  • Love Confession: She outright confessed to Naomi and gets rejected.
  • Loving Bully: As much as she is attracted to Iori, she cannot help but express herself by teasing and/or trolling him, especially during his stint as her junior at a part-time job.
  • Not So Different: She actually gets along with Iori and later Kouhei because they're all Green-Eyed Monster that can't stand others being in happy relationships.
  • Ship Tease: She has a subtle attraction for on Iori, who doesn't recognize it partly due to his obliviousness, partly due to his disgust with her Alpha Bitch antics.
  • Unfortunate Name: She never gives out her surname because "busu" can also mean "ugly" and "poisonous".
  • Villain Respect: She tells Iori in more than one occasion that she loves the fact that he's not shy about suggesting physical violence, blackmail, and other underhanded methods to deal with problems.
  • Vitriolic Best Buds: While she and Iori fight like cats and dogs, they do get along well underneath it all.

Other Characters

    Shiori Kitahara

Iori's little sister, Shiori is in her third-year of middle school and comes to Grand Blue to visit her brother just before summer vacation starts.

  • Annoying Younger Sibling: Iori doesn't exactly appreciate her company, to say nothing of her personality, especially when she visited at a most inopportune time, just when he was preparing to continue his diving licensure exams at home. This, and a desire to escape the family business, is one of the reasons he fled for Izu.
  • Aw, Look! They Really Do Love Each Other: While Iori knows almost nothing about her, he cares about her enough to know that she, like him, doesn't want to inherit the family inn. Likewise, Shiori secretly misses her brother despite resenting him for leaving her.
  • Big Brother Attraction: Double Subvertedon one hand, she despises Iori for fleeing the family ryokan and leaving her to fend for herself when it's supposed to be his responsibility as her older brother, no matter how much of a dunce their parents and she thinks he is; on the other hand, when she has to leave home Azusa notes that she wanted to enjoy the same kind of freedom Iori enjoys, suggesting she doesn't think so little of him.
  • Bitch in Sheep's Clothing: She pretends to be a Yamato Nadeshiko when she's actually a Manipulative Bitch hoping to deter her brother from running away to college and return home to inherit the family ryokan because she doesn't want that duty herself.
  • Celebrity Resemblance: Kouhei admits to Iori that part of his infatuation for Shiori (besides being a little sister) is her resemblance to "Kirara Yoimachi", a kindergarten character from another Show Within a Show, Blue Spring.
  • Cuteness Proximity: Many people finds her adorable since she resembles the anime character "Kirara Yoimachi". This unfortunately leads her to be stalked by a group of perverts of both genders.
  • Death Glare: She can pull off some amazing ones.
  • Everyone Has Standards: She knows a pervert when she sees one, as evidenced when she realizes Carina has set her sights on her, and is rightfully horrified at Chisa's obliviousness towards Nanaka and Carina's lustful overtures, going so far as to immediately hand Iori his passport, partly so he can join the freshmen's upcoming Palau trip, partly to keep him away from Carina who has expressed an intention to marry Iori in order to get close to her.
  • Hidden Depths: Even though she claims that she neither admires nor cares for her brother that much, it's shown that even with her hidden agenda, she loves Iori and misses him considerably, and is markedly upset, not only at being left to inherit the ryokan, but also because he left her alone in the first place.
  • Hopeless with Tech: Lampshaded by Iori, who claims she doesn't know how to operate a computer to send an e-mail, given she still writes letters via pen and paper. Averted when a stuffed animal she sends to him in the mail is a sophisticated camera/microphone receiver she built to monitor Iori's activities.
  • I Just Want to Be Free: Like Iori, she also doesn't want to run the family ryokan, but was forced to by their parents after Iori fled for Izu. Being forced into an unwanted position partly fuels her resentment towards Iori.
  • Jerkass Realization: While she is still determined to force Iori to return home, upon seeing in high spirits going into a diving session (where he also finally gets his professional license), as well as being invited by Nanaka to also try out scuba diving for herself, she begrudgingly admits that she has gained some insight as to why Iori chooses to stay with their uncle and cousins at Izu.
  • Just a Kid: During Iori, Chisa and Kouhei's trip to their family-run ryokan, while reviewing surveillance footage of Iori protecting her from Kouhei and two male German otaku, Shiori complains that she's no longer a child her brother needed to protect.
  • Kimono Is Traditional: Played with; she might be seen wearing it at nearly all times, but she hates it and only does so to maintain her Yamato Nadeshiko facade to her family.
  • Not So Different: Oh, she's Iori's sister alright! It's shown that Iori's underhanded methods to deal with his problems are shared by his sister too.
  • Please Don't Leave Me: While she's upset at being left to inherit the family ryokan, it's also implied that she's more upset at the fact that her brother left her.
  • Shared Family Quirks: She's actually quite a bit like Iori from their similar expressions when their plans fail, and their glorious death glares.
  • Tsundere: Her feelings for brother is depending on the situation. She pretends to be a brocon in front of others but behind everybody's back, she is a Manipulative Bitch who tries to get Iori to their family inn whatever it takes. Deep down however, she actually loves her brother.
  • What the Hell, Hero?: She does not appreciate the fact that Iori left her behind to inherit the family business.

    Kaya Mizuki
Voiced by: Nana Mizuki

An Idol Singer and voice actress of Kouhei's favorite character Rarako-tan.

  • The Cameo: In-Universe, Kaya is one of the waitresses at the maid cafe during the Oumi Festival. Hilarity Ensues when Kouhei realizes too late the day after Kaya's concert at the university that he was working at the cafe alongside his idol before leaving to help set up the concert.
  • Given Name Reveal: "Kaya Mizuki" is her stage name; she's Kanako's older sister Maya Iida.
  • Idol Singer: She is popular among the otaku with Kouhei being obsessed with her.

    Naomi Otaya

A high school student working as a chef at Iori's part-time job.

  • The Ace: Not only does he got the looks to be a major ladykiller (in spite of his own crush on Iori), but he's also pretty accomplished for a twelfth-grader, being the club leader of his own diving club with Ryuujirou even stating he's around instructor level, as well as being the head chef of a family restaurant.
  • Bishounen: Kouhei has nothing on this boy - he's so pretty it's easy to think he's a girl.

    Mrs. Kotegawa 

Chisa and Nanaka's mother and the manager of Dophin diving pad at Palau, where Chisa, Iori, Kouhei and Aina work part-time during summer break.

  • Adult Fear: Her objections towards Chisa's dream of becoming a diving instructor is rooted in their shared history of barotrauma-induced hearing problems on their left ear due to diving, with her being completely deaf on one ear and Chisa starting to show the same symptoms.
  • Ambiguously Brown: Like her husband, Mrs. Kotegawa has dark skin while her daughters have light skin tone.
  • Anger Born of Worry: She is against Chisa becoming an instructor because she became deaf on the left ear due to diving, and Chisa happens to have the same problem.
  • Cool Shades: Wears one around even during late evening.
  • Cruel to Be Kind: She balks at Chisa's dream of being a diving instructor with her poor social skills, though only because she fears Chisa would also become permanently deaf on her left ear much like herself.
  • Disappointed in You: Mrs. Kotegawa is rather disappointed in Chisa for her poor social skills.
  • Don't Call Me "Sir": When Iori refers her as "Auntie" during working hours, she ignores him until he calls her boss. Subverted as it turns out she isn't ignoring him but couldn't hear and only hears him after she turns her head.
  • Fantasy-Forbidding Father: She dismisses her daughter's dream of becoming a diving instructor. This is due to the fact that she became permanently deaf on her left ear due to diving, and her daughter is also starting to exhibit the same signs.
  • Ill Girl: She's deaf on the left ear due to barotrauma borne from all her diving.
  • Innocently Insensitive: Turns out, she wasn't trying to be cold to Chisa; due to her deafness on her left ear, she literally could not hear a significant part of Chisa's interactions, and combined with her stoicness, left a negative view of herself on Chisa.
  • Missing Mom: She is missing for most of the series until Iori and his friends visit to Palau. In fact, both her daughters haven't met her for some time.
  • Only Known by Their Nickname: She is only referred as "boss" by the her staff and "Auntie Kotegawa" by Iori. As of this writing, her first name is unknown.
  • Oral Fixation: She tends to have a lolipop in her mouth.
  • Parents as People: She hasn't been much of a presence in Chisa and Nanaka's life, and in what few interactions she and Chisa had during the freshmen's Palau trip they don't exactly get along well, largely due to a combination of their similarly introverted personalities and her fear that Chisa is going deaf on her left ear like her.
  • Small Role, Big Impact: It's implied that most of Chisa's insecurities about herself come from her frayed relationship with her mother.
  • Strong Family Resemblance: Aside from their cold personality, Chisa also inherits her mother's eyes.

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