The main character, a 19-year-old (one year older in the anime) who has failed the entrance exams for the prestigious University of Tokyo twice. During his third attempt, he is asked by his grandmother to become temporary manager of the Hinata House, a hot springs inn, while she is away on a trip — except she forgot to tell him that it has since been converted into an all-girls' dormitory.
Accidental Pervert: All the time! The fact that he lives in a) a female-only dormitory and b) said dormitory is also a hot spring, the odds are completely against him. It even happens at his wedding!
Adorkable: Despite being so bumbling, he always tries to keep his high spirit and kind heart, which makes him quite endearing.
Childhood Marriage Promise: To a girl whose name he can't remember, back when they were very young. Mutsumi loved him, she gave him his first kiss, she's the one to tell him tells him about the lovey legend of Tokyo U, So sat by like a not on a log while Mutsumi and Narusegawa play rock scissors paper to decide who gets him. Because Mutsumi is friends with both Keitaro and Naru, Mutsumi lets Naru win because Naru liked him too.
Clueless Chick Magnet: He fails to realize that there are five different girls (Naru, Shinobu, Motoko, Mutsumi and Kanako) in love with him at the same time, no less.
Determinator: It doesn't matter how many times he fails the entrance exam. He WILL fulfill his promise!
The main female character, a 17-year-old top scorer at mock exams and a resident of the Hinata House. While initially disdainful of Keitaro, his poor academics and his accidental perversions, she starts to fall for him, even as she starts to remember that, like Keitaro, she also made promise with a boy whose name she cannot remember to enter Tokyo U together with him.
Blind Without 'Em: An entire episode centered on her traveling with Keitaro while being unaware that he was there because her glasses were broken.
Character Development: Finally kicks in by the last few volumes of the manga, as we learn her reasons to be the way she is.
Character Exaggeration: The anime ups her tsuntsun, to the point that some people are warned about her bitchiness before they start watching the series. It's really not helped by how her Character Development doesn't start to become apparent until the last two episodes of the first series.
Broke down in tears twice in Chapter 17 after failing her very first entrance exams.
Thrice during the Paraklese Island arc: while wondering if she failed the second time around, again (this time in joy) when she learns she (and Keitaro as well as Mutsumi) did pass, and then she fully breaks down crying while blaming herself and her sudden diarrhea for missing the last plane to Tokyo on the deadline of submitting her and Keitaro's proof of passing the exams (and she cries for Keitaro instead of for herself). Although it turns out they've crossed the International Date Line and thus had an extra day left.
Domestic Abuser: Her beatings of Keitaro got more and more ridiculous as the series advances (although it's toned down in the last volumes of the manga). Cranked Up to Eleven in the anime.
Ill Girl: She has asthma, and as a little girl it was WAY worse. Her Tokyo U promise with Keitaro and Mutsumi happened during a time when her asthma attacks got so severe she was sent over to the Hinata House in hopes that its climate would help improve her health.
I Know Karate: Seta taught her Jeet Kun Do, although she's not as proficient as Motoko with her kendo or as Keitaro when he also learns.
I Want My Beloved to Be Happy: She tries to do this towards Keitaro and Mutsumi when she's convinced that Mutsumi was Keitaro's "Promise Girl". It doesn't last.
Naked on Arrival: In the manga, she's introduced when she goes to take a bath in the hot springs.
Pervert Revenge Mode: A few times in the series, Keitaro accidentally walks in on Naru changing her clothes. Understandably, she punches him.
Ronin: Failed to get into Tokyo University on her first try.
Tsundere: She's completely unable to be honest with herself, so it's almost impossible for her to even come to terms with the fact that she has feelings for Keitaro and spends almost the entire series denying it and beating him up. Even after she accepts it, she still turns violent at the drop of a hat without giving Keitaro a chance to explain himself. Taken Up to Eleven in the anime thanks to Flanderization.
A newcomer to the Hinata House and, like Keitaro, is a 20-year-old Ronin, though her failures stem less from being an academic underachiever than from her clumsiness and tendency to faint at the most inappropriate times.
The Ace : Especially To Naru; Mutsumi repeatedly outdoes Naru in various tasks not intentionally or meanly, she just does.
Talking to Naru about the letter from Keitaro. Mutsumi:"Don't you remember…I told you about it just a few pages ago."
Genius Ditz: She's actually smarter than Naru and didn't even have to study nearly as much.
Hair Antennae: Mutsumi and Naru share this trait. And unlike Naru, Mutsumi is indeed rather ditzy.
Ill Girl: To the point of fainting from anaemia. By the end of the manga her symptoms seem to be gone.
I Want My Beloved to Be Happy: To the max for Keitaro. She does this so many times for Keitaro and Naru, even though Mutsumi has loved Keitaro since they were little. Mutsumi later admits to Naru that she still loves Keitaro.
Mistaken for Pregnant: When Keitaro and everyone else woke up to find Mutsumi sleeping naked in his futon with him. Mutsumi's comments about last night being great and the test being "positive" had even Keitaro himself thinking he knocked her up. Actually, she just fell through the hole in Naru's room, decided to sleep where she landed, and was referring to Keitaro's help in studying. When Keitaro expresses mild regret about the misunderstanding, of course, he's chased off by Naru (vowing to personally neuter him).
Out of Focus: Particularly jarring considering she is a main character just as Keitaro and Naru are.
Romantic Runner-Up: She doesn't get Keitaro, although she stood a very good chance since she treats him rather well and could have also been the promise girl from when they were kids.
Romantic Two-Girl Friendship: Accidentally with Naru in many scenes, apparently without even realizing it. Turns out she, like Keitaro, did make a promise with Naru to enter Tokyo U together, after asking Naru if she remembered the promise made to Keitaro (Naru had forgotten) a very young Mutsumi made this promise to make sure Naru remembered to keep the promise made with Keitaro.
Ronin: Like Keitaro, she failed her entrance exams two years straight, though not due to poor academics, but due to mishaps (such as failing to write her name) and her condition (fainted just before the exams started). She ultimately passes at the same time as Keitaro and Naru, and breaks the news to both of them in Paraklese.
A 19-year-old member of the Hinata House and a freelance writer. Trickster, troublemaker, and professional layabout. In contrast to her best friend, Naru, she revels in her assets, and is not above using every scheme in the book to get her way, or at the very least make the house less boring.
Meaningful Name: Akamatsu intentionally gave her face "fox-like" features to drive the point home.
Out of Focus: Even Akamatsu himself admits with regret she is an underused character with a potential for a lot more backstory. The anime averted this by giving her a larger part to play in the Keitaro/Naru/Mutsumi triangle and exploring further her crush on Seta.
The Resenter: She is Naru's best friend, but deep down she resents that Naru was always way more popular with guys than her.
Which is to say that she resents the assumptions made by the guys concerned, not Naru.
Unreliable Narrator: Kitsune starts explaining Naru's past, and says that Naru and Seta were in a Teacher/Student Romance at the time. She then immediately states "If that had happened, it would have been interesting."
Shinobu, the youngest member of the House at 13 years old, decided to live on her own due to her parents' divorce and her refusal to choose between which parent to live with. She and Kaolla are the youngest residents of the House, being the mild-mannered counterpart to the energetic Kaolla.
Cannot Spit It Out: So many times she could confess to Keitaro, but she's too shy to do so.
A 15-year-old kendo practitioner and member of a family with their own style of kendo. Probably the least stable of the dorm residents, as she has severe insecurities about men, her own sense of femininity, her family, and turtles.
Action Girl: Stands out all the more in a non-fighting series.
Clothes Make the Superman: Episode 12, which centers around her, has her unable to focus her ki after her usual outfit ends up in the wash. It's subverted by the end after Keitaro talks some sense into her.
Covert Pervert: When she spaces out while studying, she has a habit of writing out extremely risque love stories.
Cry Cute: Twice. After Tsuruko curbstomps her and breaks her katana, deeming her too weak to carry on their school's legacy; and after Keitaro tries to comfort her. In the manga, it comes in response to Keitaro's request for her to act more feminine. She keeps failing in her attempts and finally breaks down in tears. Realizing that he's made Motoko absolutely miserable in a gender stereotype that isn't hers, Keitaro calls off the request and asks her to be herself.
Dangerously Short Skirt: Wore one as part of a school uniform in the anime, just before Kanako (disguised as Naru) felt her up.
D-Cup Distress: She complains that since her breasts are so big and still growing it becomes difficult for her to wrap her sarashi.
Heir to the Dojo: Since her sister gave up the life of the sword to marry, Motoko is the next in line to inherit the Aoyama family's Kyoto dojo as she is not married.
Heroic BSOD: Both times after being defeated by Tsuruko.
Hidden Depths: Her overtly masculine behavior comes from her misinterpreting how life went down for her big sister after she got married.
Hime Cut: Her hairstyle fits her traditional upbringing and samurai-like personality.
Huge School Girl: During her brief stint trying to be more feminine, she noticed that she was actually taller than Keitaro, ramping up her already harsh insecurities about her femininity. Of course, coming from a long line of female warriors it follows that her height would be above average - and Keitaro's not that tall...
Primal Scene: A databook released post-series explains that she developed her dislike of guys and fear of turtles when she accidentally walked in on her sister and brother-in-law in an intimate position. The Japanese word for the head of a penis is written with the characters for "turtle" and "head." Do the math and...
Matron of the Hinata House and Keitaro's 29-year-old maternal cousin and aunt, having been adopted by their grandmother after the death of her own mother, who was the older sister of Keitaro's mother.
Christmas Cake: She hates being reminded of her single status and called "Oba San". In the end of the manga, she marries Seta.
Covert Pervert: Implied — whenever one of the girls is wearing a weird outfit, chances are it came from Haruka's closet, which apparently includes a frilly Victorian dress and a Meido outfit. Add to that her skill with (and ownership of) a whip...
Cry Cute: Cries while accepting Seta's marriage proposal.
Generation Xerox: Her rough treatment of her childhood friend Seta is mirrored by Naru towards her cousin Keitaro.
Honorary Aunt: Towards her younger cousin Keitaro. Although, due to adoption, she actually is his aunt as well as his cousin.
Romantic False Lead: His introduction, which he lampshades: "I thought about swooping down in a helicopter to pick you up, but that's such a clichéd way to introduce a wealthy rival, don't you think?" Soon after his introduction, he devolves from this into more of a Butt Monkey.
Kaolla's "brother" who is actually her cousin. He is the king of their nation, and in the tradition of their people, he has to serve in the military for a period of time. Before that happens, he is required to marry. Once when Kaolla was a very young girl he promised to marry her, but then he realises that she only sees him as a brother, whereas Amalla is in love with him. He marries Amalla instead.
Ascended Extra: He is only mentioned in the manga, but appears in the anime.
A young girl of the island Pararakelse. Her grandfather was once an acquaintance of Seta, and after her grandfather's death, Nyamo decided to work with Seta on his excavations to continue her grandfather's work.
Identical Stranger: She's identical to Shinobu, except for her darker skin and longer hair.
Innocent Fanservice Girl: The girls try to cheer Keitaro up in the hot springs. Though the plan had everyone in bikinis, Nyamo doesn't get it and is completely nude.
A timid girl who appears in the epilogue. Ema hopes to get into Tokyo University so she can improve her life and have people notice her. Due to her poor grades, she left home in order to live at the "legendary" Hinata Inn, which is believed to have the power to get anyone into Tokyo University.
Accidental Pervert: She is confused with a peeping tom by Shinobu when she first arrives to Hinata Inn.