As a child, Keitaro made a promise to a girl, based on a story the children overheard, that two people who love each other will live "happily ever after" if they enter 'Todai'.
[1] Keitaro is set on gaining entrance to Todai to fulfill the promise he made to his childhood friend, even though he cannot recall her name or her face.
After failing to pass the entrance exams on his second attempt, Keitaro's parents seem unwilling and/or unable to let him continue living at home while he studies.
[3] To compound the confusion, Keitaro's Aunt Haruka mistakenly assumes he has passed the entrance exams and is already a Todai student; consequently, the residents agree to let him stay.
When he fails to correct her, and they discover that he is lying, Keitaro is forced to leave until Haruka announces that his grandmother has transferred the ownership of the inn to him.
Of all the girls at the Hinata Inn, she is most often the unwilling and unwitting victim of Keitaro's clumsiness, which often results in her ending up naked, partially clothed, or placed in any number of compromising positions.
Mutsumi also has anemia which caused all sorts of situations, but she is actually extremely resilient, having a strong vice-like grip even when she first appeared and just like Keitaro is seemingly able to recover quickly.
Yet, she still fails to get into Tokyo University because, among other things, she forgets to write her name on the test paper, causing her to obtain a grade of Z, or fainting from her anemia.
Tamago or Tama for short was originally Mutsumi's pet turtle, That she gives to Naru after she and Keitaro help her get back to her home in Okinawa.
Shinobu decides to leave home and live at the Hinata Inn because of the marital problems between her father[9] and her mother, whose names are never mentioned (in the anime).
She is often barefoot and dressed in nothing else except her heavily altered school uniform in almost every anime episode and manga chapter, or some otherwise scant costume.
To top her personality, she is above all extremely hyperactive, usually hugging (or, in Keitaro's case, kicking) someone, trying to eat something, especially bananas and Tama-chan, or inventing some potentially dangerous new device.
Her ambition is to return to her homeland to start a computing school and transform her country into a technological power that will rival, and eventually conquer, Japan.
Her room in the Hinata Inn is a faithful recreation of Molmol's natural habitat, complete with banana trees, a waterfall, and a small lake, along with a high-tech laboratory.
She also is a bit greedy, as seen in a flashback sequence when she tells Keitaro about how much Naru's innocence ended up dismissing possible boyfriends; Kitsune was shown to encourage her to date a rich boy just because of his wealth.
She also knew the location of the Liddo-Kun shaped time capsule Keitaro, Naru, and Mutsumi buried beneath the sandbox when they were children.
He's clumsy — especially with women, and his relationship to Haruka seems to be similar to Keitaro's relationship with Naru (i.e., both get physically thrashed around by their respective female and are married to them eventually as well as both of them apparently being invulnerable to just about everything), a terrible driver (his arrival often heralded by his van crashing down from the sky), and an exceptionally skilled martial artist, all partly due to his complete lack of self-preservation.
Sarah is left for a time in the care of the Hinata Inn residents by Seta, as he feels that travelling the world with him on expeditions would be too disruptive to her childhood.
Sara possesses no unusual talents or skills, save for a certain gift for Jeet Kune Do learned from Seta, though she is adept at bashing Keitaro on the head with priceless artifacts often pulled out of nowhere.
When the girls (except for Naru) start to suspect Kanako's feelings for Keitaro and why she forces them to be mean to him, they form the "Hinata House Liberation Front" in order to reclaim the Inn.
Kanako, Naru and Mutsumi form the Hinata-rōshi (日向浪士, translated as "Hinata Rebel Alliance") to reclaim it under the name Urashima Kanako-no-suke (浦島可奈子之助).
Haruka let him try to work off his debts at a very low pay rate of $1.50 an hour; from that point on, Kentaro became an occasional background character and plot device, like giving advice to Keitaro and supporting his feelings for Narusegawa.
She also appears in the last volume of the manga, seen standing off on the side with the rest of the residents of Hinata Inn, while Naru is chasing Keitaro down after accidentally undressing her.
Like her sister, her sword techniques are centered around the destruction of evil, using ki attacks with her slash, some of them hurting only evil beings, but mostly slashing through rocks, people or larger objects (like a sports car, driven by Kentaro, that was 'accidentally' about to run over Keitaro – in the manga, it was a truck whose driver was drunk and sleepy) as if they were made of paper.
Like Shirai, Haitani's primary interests lie in women, relying on his charm to attract female attention, but to no avail.
A timid 15-year-old girl who dabbles in conspiracy theories, 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.
In awe of all the beautiful girls living at the Hinata Inn, she accidentally steals Naru's wedding veil, tripping a theft alarm in the process.
Kaolla is very interested in researching it, and Sarah calls him "witch cat" after knowing about Kuro's unusual abilities to fly (using its oversized ears) and talk.
As a footnote, Ken Akamatsu explained in his notes in Volume 14, page 148 that Tama, Kuro, Shippu, and Leon were inspired by the "Four Legendary Animals of Japanese Mythology".