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When he was four years old, Keitaro met with and made a promise with an unknown girl around his age that the two would enroll into Tokyo University together. He never knew the girl's name nor could he recall what her face looked like. After failing for two years to make it as a university student, Keitaro was kicked out of his parents' home for his persistence at continuing to try to enroll to Toudai. Hoping to take up residence at the Hinata Inn apartments that his grandmother owned, Keitaro was unaware that the inn was reformed into an all-girl's dormitory. He learns this the hard way after getting into an embarrassing predicament in the inn's hot springs with Naru Narusegawa. Following this predicament, Keitaro would learn that he has become the manager of the inn as his grandmother has retired from the job.
Keitaro was created as the main lead for Ken Akamatsu's harem manga series, Love Hina. Originally, his last name was supposed to be Akaishizawa and would be depicted as having a more loudmouthed personality. In early developments of the manga, Keitaro would have wanted to sell the inn off due to its age and the problems caused by many of the tenants. He has appeared in all anime adaptations of the series, which include the TV anime, Christmas special, Spring special and Love Hina Again OVA series. He is voiced by Yuji Ueda in Japanese and Derek Stephen Prince in English.
Keitaro stands at 171 centimeters tall (about '5'7"), having brown hair and eyes. He is normally seen wearing glasses, as his vision is otherwise poor without them. At the start of the series, Keitaro is 19 years old in the manga and 20 in the anime.
Keitaro has an honest and hard-working personality as he has the drive to help others solve their problems, usually at the cost of his dignity and personal safety. Because of his plain-looking attire and failure to make it into Tokyo University up to three times, many seen him as an uninteresting failure of a man. He is regularly a victim of various misunderstandings and accidents with the inn residents that lead him to be comically beaten up by the more short-fused residents of the inn.
- Naru Narusegawa- Naru is Keitaro's love interest throughout the entirety of the series, as he suspected the possibility that she might be the promised girl he met from years ago. She would frequently beat Keitaro up for the slightest transgressions he may have unintentionally caused. Despite this negative aspect to her personality, Naru is willing to tutor him in preparation for entrance exams and lend him advice. While outwardly expressing that Keitaro wouldn't be the ideal man she would love, Naru finds herself developing romantic feelings for him and her stubborn denial of these feelings prove to be the major obstacle preventing their relationship from developing further.
- Mutsumi Otohime- This seemingly frail and naive young woman is encountered by Keitaro when he runs off to Kagoshima after failing his third exam. The two developed a high compatibility for one another due to their similar personalities and backgrounds with both having failed for multiple years to make it into Toudai and trying to make a childhood promise with someone they knew from their past. While easily making friends with Naru and Keitaro, her presence leads to a love triangle to develop with the three as she takes part in studying with the two for entrance exams and all three learned that they were all at Hinata Inn at the same time that the promise was made.
- Motoko Aoyama- Motoko was the most aggressive in her opposition of Keitaro's presence at the Hinata Inn due to her hatred of men and would often attack him with her sword when he was caught in a misunderstanding with any of the other female residents. Over time, Motoko would become used to Keitaro's presence at the inn and gradually mellowed out of her aggression toward him. Keitaro would assist her in overcoming some personal issues she dealt with such as coming to grips with her femininity, improving her strength as a warrior and settling hostilities with older sister Tsuruko. As the series progressed, Motoko would come to develop romantic feelings for Keitaro.
- Mitsune Konno- The resident slacker at Hinata Inn is known for her mischievous streak toward Keitaro as she often sets up situations that led him to be a victim of Naru and Motoko's wrath. Despite her troublemaking ways though, Mitsune does usually offer advice to Keitaro over ways to get Naru's attention, especially when things are complicated with the presence of Seta returning to Naru's life.
- Kaolla Su- Keitaro started off on rather shaky terms with Kaolla as the girl usually participated in the pummeling he suffered from the other inn residents and would use him as a guinea pig to test her inventions on. Despite her behavior though, Kaolla would gradually develop romantic interest in him due to his similarities to her step-brother and desired to move with him and the Hinata Inn and Molmol so they could all still be together.
- Shinobu Maehara- Keitaro has a sempai-kohei style relationship toward Shinobu, being mostly oblivious to the one-sided crush she has towards him. He does care enough for her to help tend to any personal problems she dealt with such as her parents' messy divorce in the anime or knowing how to prepare for tests in the manga. Shinobu is the only resident who doesn't belittle or harm Keitaro for the majority of the series.
- Seta Noriyasu- Despite some jealousy that Keitaro may have toward him in terms of earning Naru's affections, Seta is a sort of inspirational figure for Keitaro in his goal to make it into Toudai as Seta underwent the same struggles Keitaro went through as he failed to get into Toudai several times before finally passing his entrance exams. Keitaro would serve as an assistant to Seta in archaeological projects and would be trained in martial arts through him in later parts of the series.
- Sarah McDougal- Seta's adoptive daughter was prone to berating and pummeling Keitaro in early parts of the series where she was introduced. Upon learning of the panda-shaped birthmark she had on her butt in the manga, Keitaro would use this as blackmail material to keep Sarah behaved until she better adjusted to him.
- Haruka Urashima- Being Keitaro's cousin and aunt by technicality, Haruka often advises Keitaro on how to handle things with the tenants at the inn, particularly his growing relationship with Naru. However, Haruka will pummel Keitaro if he makes mention of her being his aunt or doing anything that would annoy her.
- Kanako Urashima- Keitaro's stepsister who would appear later in the series to attempt driving out the residents living at Hinata Inn. When both were kids, Keitaro and Kanako made a promise to get married and own the inn. While Kanako literally took this promise as being legitimate, Keitaro was mostly oblivious to the implications of this promise due to how young he was. He would find himself caught in the middle of Kanako and Naru's heated rivalry over him, the events leading Naru to come to terms with her feelings for Keitaro.
- Nyamo Namo- Nyamo was a young girl at Pararakelse Island that Keitaro assisted in trying to track down the whereabouts of a lost turtle civilization in her grandfather's memory. Much like Shinobu, Nyamo would develop a crush on Keitaro.
As the manager of Hinata Inn, Keitaro would often find himself at odds with many of the female residents as he would be victim to their comical pummeling of him if caught in some sort of predicament with any of the residents. Regardless of how violent the pummeling may be, Keitaro had been able to survive his scuffles with the girls without any harm, which would comically lead the residents to assume that he might be immortal or invincible as a result.
Throughout the series, Keitaro struggled to pass his entrance exams to make it into Tokyo University and meet his promised girl as it took four years for him to accomplish this. He would find himself facing a love triangle with Naru and Mutsumi, as both girls were with him as children years ago and one of them was whom he made the promise with. By the end of the series, he would fall in love with Naru, study up on Archaeology during his time at Toudai and marry Naru in the series finale.
Details on whom Keitaro's promised girl was differ between the anime and manga. In both instances, Mutsumi was revealed as the girl that Keitaro recollected from his childhood yet was revealed to have not made the promise with him. The anime would never reveal who the promised girl was. In the manga, it was revealed that Naru made the promise with Keitaro. She was introduced to him and Mutsumi when the two were playmates at Hinata Inn and Mutsumi winded up talking Naru and Keitaro into making their promise together.