• Adults Are Useless: Other than Masa and Ren.
• Art Shift
• Battle Aura
• Bizarre Alien Biology
• Curtains Match the Window
• Cute Monster Girl: Female mermaids.
• Involuntary Shapeshifting: Young mermaids cannot maintain their legs if they get wet.
• Love Dodecahedron
• Make Me Wanna Shout: Female mermaids can use their voice as a weapon.
• Mermaid Problem
• New Transfer Student
• Our Mermaids Are Different
• Theme Naming: A lot of mermaids have the name of an important body of water in their family name and a celestial object as their given one.
• Why Did It Have To Be Cats: Most mermaids (since they are technically fish). Most of them get over this fear in time. Most of them...

• Innocent Cohabitation
• Official Couple
• Sleep Cute: In episode 13.

Nagasumi Michishio

• Action Survivor - Nagasumi only becomes badass because everyone is trying to kill him.
• Ahoge - Nagasumi. It seems to act as some sort of threat-detecting radar for him.
• And This Is for...: Sun, naturally.
• Attack of the 50-Foot Whatever: In Episode 11 after drinking a health drink that was only meant for mermaids. The affair makes him Genre Savvy because he never trusts a mermaid drink (or drink offered by a mermaid) again.
• Badass Normal: He's a normal human that lives with superpowered mermaids. Most of them try ( and fail) to kill him.
  • Badass Abnormal: The mermaids note that he becomes "progessively less human" as the series progresses. This involves: resistance to mermaid singing, a Spider-Sense, and the ability to trigger a Super Mode without Sun's Song of Heroes.
• Batman Can Breathe in Space: Nagasumi gets blasted to the surface of the moon. They didn't even WANT to try to bring actual physics into play for that one.
• Butt Monkey: BIGTIME. He's the source of almost all the show's slapstick humor, with Chimp filling in the rest.
• Cherry Blossoms: Episode 26 shows you can get them for playing too much with Setouchi.
• Fan Nickname: NaGARsumi.
• Nice Guy: Matures into this for Sun. At the start he was kinda of a brat.
• Not What It Looks Like: Gets dragged into this kind of situations A LOT. It doesn't help that drying off a mermaid's legs (the only way they can resume human form) looks like rubbing her thighs.
• Only Sane Man: "Am I the only one who thinks this is a little weird?" Everyone else takes the weirdness in stride.
• Ordinary High School Student: He is normal at the starts of the series but is clearly part of the Seto Gang by the end.
• Radial Ass Kicking: He does this during Episode 8 while affected by Sun's Song of Heroes.
• Sacred First Kiss: And the show WON'T let you forget it.
• Secret Keeper: For Sun and later the mermaid world as a whole.
• Serial Escalation: His voice reached Sun despite her being drugged and immediately afterward activated the Super Mode without Sun's Song to power him up. "I don't need it. Just being near you fills me with unlimited power!"
• Spider-Sense: By the fifth episode, he's dodging Maki's attacks in his sleep. He can also use it to detect whe someones about to break The Masquerade.
• Suplex Finisher: He does this to Shark Fujishiro when he is attacked during the festival.
• Takahiro Mizushima
• Todd Haberkorn
• Took a Level in Badass: Pretty weak in most episodes but steps up in the finale.
  • Bullet Catch: The Oh Crap face it inspires in the Big Bad is fantastic.
• Unwanted Harem: Sun, her Mom (though she's more of a Shipper on Deck for her daughter that's having fun at his expense Lunar, Mawari, and Class Rep.
• Wrestler in All of Us: The famous "Sharp Suplex".

Sun Seto

• Badass Adorable: She's sweet and clumsy but also her father's daughter.
• Betty and Veronica: She is the Veronica to Mawari and the Betty to Lunar.
• Catchphrase: The alternate way of reading 'Ninkyo'(Chivalry)...is 'Ningyo' (Mermaid)!
  • "Because honor among thieves is honor under the sea!" in the english dub.
• Cherry Blossoms
  • She gets it from her mom.
• Dojikko: Especially when she's nervous. Quite a problem when water turns her back into a mermaid.
• Distressed Damsel: In the climax.
• First Girl Wins: First girl introduced. Mawari is first chronologically.
• High Pressure Emotion
• Hot Chick with a Sword: In the words of her fiance 'Once she gets a sword in her hands she's like a completely different person." More focused and less culmsy.
• Instant Fan Club: Latent until Lunar showed up. Then suddenly she had her own militant faction that was willing to kill her fiance for more of her attention.
• Mafia Princess
• Magic Music: She demonstrates the power to put people to sleep and send Nagasumi into Super Mode.
• Megaton Voice: Unlike most practicioners of this trope, though she does it for the same reason she feels horribly guilty afterward and apologizes.
• Punny Name
• Seashell Bra - most notably in her first appearance.
• Song Of Courage: One of her songs is the "Song of Heroes' which can empower Nagasumi.
• Spell My Name with an S - Sun/San
• Testosterone Poisoning: In Episode 22. Between the "Military officer" beard and the deliquent outfit and proclaiming Real Men Eat Meat its both cringe worthy and funny.
• Yamato Nadeshiko: She's big on family tradition and the family business, as well as being very devoted to her fiancé.
• Zettai Ryouiki: Present when wearing her school uniform.

Mawari Zenigata

• The Beast Master: She can summon a horde of cats with a whistle. These are beasts in the eyes of merfolk. Also subverted because though she can summon them she can't really control them.
  —Chimp: I had no idea Mawari had gotten so good at controlling caaatttssss!
• Catchphrase: "Must Mawari teach you the rules of society?"
• Caitlin Glass
• Control Freak: She's called the "Demonic" head of the Disipline Comittee for a reason
• Genki Girl: She pursues her "Sense of Justice" with great energy!
• Lawful Stupid - Played for comedy. Mawari, who desired to be a "good cop", tries to be Lawful Good, but due to her overzealousness, she veers towards this instead.
• Sympathetic Inspector Antagonist - Even shares the Lupin III inspector's surname.
• Unlucky Childhood Friend: For Nagasumi. There was even a flashback of them playing.

Hideyoshi Sarutobi (Saru)

• A Storm Is Coming: Plays this trope to perfection in Episode 8 in his 'hermit' mode.
• Barehanded Blade Block: He attempts one in Episode 17 but fails.
• Bromantic Foil
• Butt Monkey: Even more so than Nagasumi.
• Expy: Of Hideyoshi from The Law of Ueki, they even share the same given name.
• Hot-Blooded Sideburns: In a preverted fashion.
• Hypercompetent Sidekick: For Kai. Interestingly, he becomes hypercompetent by becoming Kai's sidekick. Otherwise he's just a preverted monkey..

Lunar Edomae

• Always Second Best: Feels this way about herself in comparison to Sun because Sun always beat her in singing contests growing up.
• Bitch in Sheep's Clothing: Her stage persona is kawaiiko to appeal to fans. If you dig beneath that she's a Jerkass, but if you dig beneath that (as Nagasumi's mom did) she's sweet again.
• But Not Too Foreign
• Disproportionate Retribution: Just because she lost a singing concert to Sun when they were both children she tries to ruin Sun's life.
• Expy: She is similar to Etna in appearance and personality.
• Has a Missing Mom.
• Idol Singer: Number 1 in Japan. She did it to prove her superiority to Sun.
• Instant Fan Club: Deconstructed. Her fan club already existed before she enrolled in Nagasumi's school. It just wasn't militarized yet.
• It's All About Me: She thinks Sun married Nagasumi to flaunt this fact in her face, disregarding the whole 'marry into the mermaid family or die' thing.
• Jerk with a Heart of Gold: She acts haughty and abrasive to everyone but there's a Nice Girl underneath it all.
• Lack of Empathy
• Mafia Princess: Her dad is the head of the Edomae gang.
• Magic Music: Her favorite is the "Song of War" Which turns its audience into mindless warriors.
• Mistress and Servant Boy: She tries to force Nagasumi into one of these relationships.
• Noblewoman's Laugh
• The Ojou: From her dad's status and her own as a pop star she's treated like nobility.
• Royal "We": The only time she ever speaks in first person is when Nagasumi teaches her a lesson in humility.
• Sakura Nogawa
• Schoolgirl Rival: To Sun over singing contests, love interest, and their fan clubs.
• Spell My Name with an S: Lunar/Luna/Runa
• Unknown Rival: For Sun, who thinks they're best friends. They're at least Vitriolic Best Buds late in the series.
• Yandere: How much it was for Nagasumi's love and how much was to one-up Sun is up for debate but she certainly had extreme measures.
  • Tsundere: Mellows in the second half of the series into this, for both Nagasumi and Sun. For the former, she'll openly flirt but violently deny actually liking him. For the latter we have this gem:
  —''We rearranged our schedule to see you, but don't get the wrong idea! Its not because we wanted to have fun with you!"
• Zettai Ryouiki: Hers are black for the added contrast to Sun's white.

Class President

• Beautiful All Along: before◊ and after◊.
• Blind Without 'Em
• Class Representative: Everyone thinks this is her name.
• Covert Pervert: When she says the Nagasumi has become more manly (episode 15), the two incidents she sites involve him being naked.
• Dropped Glasses: In episode 15.
• Everyone Calls Him Barkeep: She is never referred to by her real name (much as she wants to be), only 'Iincho'.
• Faux Yay - The Class President pretends to be a lesbian to overcome her shyness.
• Meganekko - Inverted in that her glasses hide her attractiveness rather than produce it. Debatably, although it looks like her character design budget was doubled for Episode 15.
• Shrinking Violet: Painfully shy, and to make it worse, the one time she build the courage to overcome it, she ended up confessing her love to Sun on actident.
• Transformation Sequence: Near the end of episode 21 into "Last Amazoness".

Kai Mikawa

• Cool Sword: One with a long history, but he doesn't get to use it until the climax.
• Daisuke Ono
• Eric Vale
• Hikikomori: He never truly went outdoors until transferring to Nagasumi's class. The term is mentioned in his joint Image Song with Saru.
• Lonely Rich Kid: He's in love with Sun because she didn't care about money or status. That and she played a weird version of house with him.
• Rich Bastard: He uses money to solve his problems. Sun calls him on this in the first episode and he gets a little better about it.
• Mistaken for Dying: In Episode 24.
• Screw the Rules, I Have Money!
• Shonen Hair

Akeno Shiranui

• Anti-Villain: Type 5. While she is a antagonist to the main pair, if she didn't do her job there would be more problems for everyone.
• Big Brother Worship: Adored her older brother and wants to be like him.
• Catchphrase: "How vexing."
  • Phrase Catcher: Was referred to multiple times as 'vexingly sexy' for an episode.
• Eri Kitamura
• Heel Face Turn: She was a villain starting out with her mermaid exam but switches to Nagasumi's side in the climax.
• Hot Chick with a Sword: Cold, driven and very good with a sword.
• Magic Skirt: During her swordfight with Sun.
• Tall, Dark and Bishoujo
• To Be Lawful or Good: As a trainee swordsman she has to obey nobles but as a good person she has to disobey one. She choose to be good.
• Tsurime Eyes
• Why Did It Have To Be Ghosts

• Badass
• Card-Carrying Villain
• Love Obstructing Parents
• The Yakuza Who Don't Do Anything
• Villain Protagonist

Gozaburo Seto

• Good Scars, Evil Scars: On his face. The good kind over the nose.
• Henpecked Husband - It becomes painfully clear in a few episodes that it is Ren who is the true ruler of the Seto Yakuza clan, not Gozaburo.
• John Swasey
• Kenta Miyake
• Knight Templar Parent /Overprotective Dad: He inflitrates Nagasumi's school and sends assassins.
• Tears of Blood: When Sun decided to live with Nagasumi.
• Why Did It Have To Be Cats: He still has a fear of cats because of an incident in his childhood where a cat bit his ear while he was sleeping and he nearly lost it due to the resulting infection.

Ren Seto

• Cherry Blossoms
• Happily Married: Despite how much Gozaburo's antitics anger her she really does love him. Case in point: when Nagasumi drank the love potion she gave him she called him 'extremely sexy'. In her eyes Nagasumi was the spiting image of her husband.
• Hot Mom
• Let Her Grow Up Dear: In contrast to her husband she is a Shipper on Deck for her daughter and Nagasumi. She's very happy about the the intimacy in their relationship. If they moved beyond hugging and hand holding.
• Only Level-Headed Person In The Cast Most Of The Time
• School Nurse: Her excuse to infiltrate the school.
• The Tease: Her favorite target is Nagasumi

Masa

• Badass in a Nice Suit
• Characterization Marches On: Due to amnesia caused by Gozaburo Seto Masa used to act and look completely different, being an easy going brother of Akeno and looked similar to her.
• Christopher Sabat
• Cool Shades
• Even the Guys Want Him: Masa-san is a sexy beast.
• Funny Afro
• Identity Amnesia: After getting punched in the head by a drunken Gozaburo, he forgets his real identity.
• Long Lost Relative: He is actually Akeno's older brother.
• Sacred First Kiss: To Nagasumi (Played for Laughs constantly in the anime).
• Soul Brotha
• The Stoic

Maki

• Beware the Nice Ones: Because, in this case, they're only pretending to be nice.
• Bitch in Sheep's Clothing: Acts like a Shrinking Violet for Sun but her true personality is crude, violent, and aggressive.
• Monica Rial
• Natsuko Kuwatani
• Poisonous Friend: For Sun.
• Shout Out: She has a visor and a 'surfing ram' attack. Hi there, Temjin.
• Small Girl, Big Gun: Well, it's not particularly big by human standards, but compared to her it's quite big.
• Verbal Tic: desu~
• Yandere: For Sun. She has Undying Loyalty for Sun and will do anything to protect her up to, and including, murdering her husband for some preceived slight.

Shark Fujishiro

• Bryan Massey
• Catchphrase: "Can I eat him/her/them?"
• Everything's Even Worse with Sharks
• Goofy Print Underwear: With little fishies on them.
• Heroic Comedic Sociopath: Nearly all his scences involve him trying to eat Nagasumi or asking if he can eat the other students. This would make him a villain if he weren't (technically) allied to Nagasumi against people like Lunar Papa and if his antics weren't played for laughs.
• Takehito Koyasu
• The Only One Allowed to Eat You: Towards Nagasumi.
• Verbal Tic: He speaks as if all sentences were questions.

Octopus Nakajima

• Curb-Stomp Battle: He loses quite easily against an enlarged Nagasumi.
• Kaiju
• Unusually Uninteresting Sight: He's usually seen sticking out of a window when at school.

Maguro, Agitaro & Furiyo

• Gonk: They look like real fish.
• Paper-Thin Disguise: One of them crossdresses in order to disguise himself and get close enough to deal with Nagasumi's cat. Unfortunately, everyone involved sees through it immediately, because it's kind of hard to disguise yourself when you look like this.◊
• Those Three Guys
• Verbal Tic: All three of them have the exact same one, gyo!

Lunar's papa

• The Ahnold
• Badass
• Barehanded Blade Block: He catches Kai's sword in two of his fingers then snaps it.
• Captain Ersatz: Of the Terminator.
• Cool Shades
• Doting Parent - For what its worth, he does try to be a good father. Now if only Luna would stop pushing him away whenever he tries to reach out...
• Everyone Calls Him Barkeep: He is only referred to as Lunar's papa.
• Eye Beams: Seen in the finale.
• Gratuitous English
• Implacable Man: His introductory episode is played like a (comical) horror movie with an unstoppable monster chasing the protagonist.
• J Michael Tatum
• Lawyer-Friendly Cameo
• Made of Iron: Not even a Kill Sat can stop him.
  • His first appearance is even called "Iron Man".
• No Social Skills: He doesn't know how to communicate with people very well outside of scaring the crap out of them (especially his own daughter.)
• Real Men Wear Sailor Fuku
• The Stoic
• Tessho Genda: Who also does the voice of the Terminator in the Japanese dub.

Nagasumi's Family

• Abusive Parents: Both emotionally and physically.
• Adults Are Useless: The parents.
• Henpecked Husband
• Hot Mom: Nagasumi's mother.
• No Sympathy: Nagasumi's parents' typical reaction to any of his many, many problems.
• Opening Narration: Provided by Nagasumi's grandmother.
• Why Couldn't You Be Different?: At many points they say they'd rather have a daughter than a son, to the point that they move their son to the attic and move Sun to his room.

Satori

• Annoying Younger Sibling
• Rescue Romance: Towards Nagasumi.
• Tomoko Kaneda

Saturn

• Badass Teacher
• Blackmail: She can use her stalking skills to blackmail just about anybody.
• Prehensile Hair
• Sadist Teacher/Stern Teacher: Tends to zigzag between the two, with more of a focus on the latter as the series progresses.
• Sir Swears-a-Lot: But only when she's teaching English.
• Yandere
• You Gotta Have Green Hair: Justified in that it's seaweed.