Mutsumi is a clumsy oaf of a girl without a lick of common sense(though perhaps more than she lets on). She is constantly having near death experiences and always carries her last will and testament with her in case she should actually kick the bucket. Her favorite things in life are watermelons, dried squid, kotatsu, kissing, and turtles. Much like some other Love Hina characters she is desperately trying to get into Tokyo University due to a promise she had made when she was younger. Despite suffering from bad luck, she always maintains a bright and cheerful attitude.
A Brief History of Mutsumi
Mutsumi's mother was an employee at the Hinata Inn, which is where the young Mutsumi met Keitaro Urashima and a sickly girl named Naru Narusegawa for the first time. The trio played together, and Mutsumi was the first to hear the fairy tale that if two people who love each other very much go to Tokyo University, they'll be together forever. Mutsumi knew the younger and shyer Naru liked Keitaro, so she gave her the confidence to make a promise with him that they would both go to Tokyo U. Keitaro left sometime later, and knowing that Naru was too young to remember her promise, Mutsumi remade the promise with her.
Time passed, and the trio would eventually forget most of their time at Hinata House. Keitaro and Mutsumi would remember making promises(something that would be a driving force in their lives), though not to who; while Naru, who was little more than a toddler, grew up and remembered nothing.
The trio wouldn't meet again until, having failed her Tokyo University entrance exam, Mutsumi embarked on a "journey of self-discovery". She running into Keitaro and Naru(though she didn't recognize them, nor vice versa). They eventually decided to travel together, and Mutsumi revealed that she was trying to get into Tokyo U due to a childhood promise. Naru recognized the similarities between Mutsumi's and Keitaro's stories, but thought little of it. As a parting gift at the end of their trip, Mutsumi gave Keitaro and Naru a flying hot springs turtle named Tama-chan to keep as a pet.
Some time later, Mutsumi came to the Hinata area to stay while she studied for the next entrance exams. She rejoined Keitaro and Naru and they formed ronin study group. It was during this time that Naru found a photograph of the three of them at Hinata House as a children. This led her to the conclusion that Mutsumi and Keitaro had made their promises to each other. Naru had grown quite fond of Keitaro, and debated whether she should tell them that they were seemingly destined for each other. She finally broke down and revealed the truth to them on the night of Christmas Eve. However, Mutsumi(not the "sharpest knife in the drawer") failed to comprehend what Naru was saying, even though Keitaro did. Even worse, Mutsumi had also grown quite fond of Keitaro. Aaand to complicate matters further, the house Mutsumi was staying in burned down, forcing her work with Keitaro's aunt Haruka and to stay at Hinata House temporarily.
Things stayed somewhat bumpy between the three of them, until Naru woke up one morning and found Mutsumi in Keitaro's bed. Naru was furious, and Mutsumi made matters worse by inadvertently insinuating she was pregnant. Common sense won the day however, when Haruka revealed that Musumi was a chronic sleepwalker. As the trio made up and walked home through the cherry blossoms, Mutsumi had a revelation. She now remembered the person with whom she made her promise, but she wanted to keep it to herself until after the exams.
On the day of the fateful exams, Keitaro and Naru met up with Mutsumi. Keitaro apologized to Mutsumi that he wouldn't be able to keep his promise to her, but he was in love with Naru now. To the shock of the other two, Mutsumi told Keitaro that he was mistaken. Mutsumi had made HER promise with a girl... Naru. She attempted to explain to Keitaro that the girl he made his promise to was his first love, but she was too young to remember. The bell rang before she could explain any further.
Mutsumi finally made it into Tokyo U after that, and she was part of the crew that went to rescue Naru and Keitaro on the island of Parakelse. Later, Mutsumi, Keitaro, and Naru visited Mutsumi's home in Okinawa where more history of the promise was revealed.
After this, Mutsumi plays a much smaller role in Love Hina. After Keitaro's adopted sister, Kanako Urashima, turned Hinata House back into an inn, Mutsumi got to pretend to be Keitaro so the other residents could practice their service techniques. She and Naru were the only residents to take Kanako's side during the Hinata Liberation Wars. Later, Mutsumi and Kitsune stowed away on Naru's "Emotional Healing Journey", though this was more an attempt to eat different kinds of food.
By Love Hina's finale, Mutsumi has become a graduate student and is still attending Tokyo University.