Let's face facts: Keitaro Urashima is what some people call a loser. He's 19 years old, has never had a girlfriend, failed his college entrance exams twice, and been kicked out of his parents' home. If hes got anything going for him it's that: A) He's generally a good guy, and B) He never gives up.
Unfortunately, Keitaro is either one of the world's biggest peeping toms, half the time it is not intentional peeping though. He is constantly walking in on women undressing, in the bath, or on the toilet. Improbable events will transpire that will inevitably lead to him accidentally groping female body parts. As punishment, he is often attacked and beaten up by the female residents (especially Naru and Motoko). His ability to endure these constant beat-downs(not mention falling from great heights, being ran into by Seta's van, and attacked by Su's weaponry) lead to several characters remarking that he must be immortal. It should be noted that although these peeping incidents are almost always portrayed as accidents, at the end of the book Keitaro admits that he likes to see Naru naked.
Love Hina is the story of how this pathetic schlub ends up with a houseful of female admirers and marrying his childhood sweetheart.
Keitaro at Home
Not much is known about Keitaro's homelife; i'ts never actually shown in the manga. This much we do know: He has an adopted sister, Kanako Urashima, and his parents own a dessert shop.
Keitaro has a rocky relationship with his parents. They've evicted him at the beginning of the manga, but later call and want him to come home. They check up on him several times through the series to make sure he's keeping his grades up. Once, they call him home and discuss his future.
Keitaro and Tokyo U
The reason Keitaro tries so hard to get into Tokyo University(ignoring all others), is that when he was younger he made a promise to the only girl who had ever been kind to him. Keitaro and the girl had heard that if two people who love each other very much go to Tokyo U. together, they will live happily ever after. Keitaro does accomplish his dream, but due to several twists of fate, is never actually shown attending any Tokyo U. classes.
Keitaro at Hinata House
The first resident Keitaro met upon arriving at Hinata House was Naru Narusegawa. This encounter would set the stage for their relationship.
Believing that Hinata House was still a hotel, Keitaro decided to relax in the open-air bath. Unfortunately, Naru walked in on him. She wasn't wearing her glasses and at first thought he was Kitsune. Of course, she soon realized this wasn't the case and she chased the strange, naked man around the house. Only the timely intervention of his aunt, Haruka Urashima saved Keitaro's hide.
Despite Keitaro's repeated accidental perversity, and despite Naru's violent tendencies, they soon became rather close(although neither would admit it). Keitaro comes to believe that Naru is the girl he made his promise to, although she later disavows him of this notion.
Their relationship becomes strained when they both fail their entrance exams. Keitaro is rather used to failure, but Naru was formerly the top student in the nation. They get into an argument, and indivually decide to go on a vacation to forget their troubles. They reconcile on the trip and meet someone who will come to play a very important role in their lives; Mutsumi Otohime.
Returning from the trip, Keitaro briefly deicdes to quit school, and also learns about Naru's past with Noriyasu Seta. This causes another rift in their relationship, which soon healed... Until Mutsumi returns.
Keitaro and Naru first meet Mutsumi while on a "journey of regret and self-discovery". She is a clumsy and sickly young woman who is constantly passing out. She and Keitaro have a lot in common, including a Tokyo U. promise. Naru even goes so far as to call her a female version of Keitaro. He and Mutsumi get along very well because they're so much alike. His closeness to Mutsumi causes some friction with Naru, even though in Keitaro's mind, he prefers Naru. Things are brought to a head, when due to a chain of events, she is believed to be pregnant with Keitaro's baby.
In the middle volumes of Love Hina, it's revealed that Keitaro, Naru, and Mutsumi all played together at Hinata House when they were younger. Because of this, and because of their similar stories, Keitaro and Mutsumi are suspected of making their childhood promise to each other. After it becomes evident that this isn't the case, Mutsumi plays a much smaller role in the story.
The first girl we come to really know at Hinata House, besides Naru, is Motoko Aoyama. She is a very serious high school student and dedicated swordsman. She also seems to hate men in general, and Keitaro in particular. She calls him weak, perverted, and unmanly. She soon starts feeling weak in his presence, but it turns out to just be a cold.
A turning point in their relationship comes when Motoko's sister, Tsuruko Aoyama, comes to visit her and forcibly took her to perform her duty as the Shinmei-ryū sword school successor. Motoko lied to her and tells her she is going to get married to Keitaro. Tsuruko finds out that she is lying and declares that as punishment, she and Keitaro must marry immediately. Tsuruko gave them the condition that if Motoko manages to defeat her within the allotted time given, the marriage between Motoko and Keitaro would be cancelled. Motoko manage to indirectly defeat her sister, but realizes that she really does have feelings for Keitaro. During the conversation with Shinobu and Motoko before Naru and Keitaro's wedding, it is hinted that she is still in love with him and not willing to give up.
Shinobu Maehara is the youngest resident of Hinata House. Despite getting off to a rocky start, she develops a crush on Keitaro and calls him "sempai". She likes him because she's really shy and sensitive, and Keitaro is always kind to her. Keitaro agrees to tutor her, and when he and Naru are fighting he even agrees to be Shinobu's "date" to the fair. She realizes that he would be happier if he wasn't fighting with Naru, and goes off to look for her. Probably due to their distance in age, Keitaro never develops romantic feelings for Shinobu.
By the series end Keitaro is more like Seta than ever, and is finally preparing to marry his promise girl.