Mimi is an immortal, just like Rin. Like Rin, she too was infected by the Time Fruit at some undisclosed point in time. Mimi cares for Rin very much and would do anything to ensure her safety, mostly because she feels indebted to Rin, for saving Mimi from being devoured by an angel during their first encounter and protecting her ever since. She works as Rin's assistant in her jack-of-all-trades business. Although Mimi past never explored in depth, we can infer from her interactions with Rin – and her extensive knowledge and technical skills – that they have been “friends” for a very long time; and, at some point or another, Rin saved Mimi from an Angel. The event lead to an eternal friendship and coexistence that continues to thrive despite the hardships they have faced to this very day.
While Rin is the procrastinator, Mimi is the girl who nags on Rin to work, most of the time. She is also more wary of Rin's flaky operations – when she pursues a dangerous situation, instead of the assignment they were paid to carry out, Mimi always tries to dissuade Rin, to no avail of course.
Mimi has a bright personality, and despite her young appearance, she is rather mature, and can definitely hold her own liquor. She is always fashionably dressed, and takes it very seriously sometimes. Mimi also has her flirtatious side, and is implied to be either bisexual or a lesbian, since she playfully hits on Rin, kisses her at times, and accepts Rin's occasional advances on her, and even having sex with the Informant's Assistant a few times with gradually less protest about it.
Mimi's job scopes include: financial management, information processing, and other miscellaneous tasks. She is also a formidable hacker, and her computer skills are obvious throughout the series. In Episode Three, she also became a skilled investor, with some of those investments evident during the caper on Hagashima Island.
Mimi's hacking skills are outmatched by Kamiyama in Episode Four and, after Rin disappears, Mimi leaves with Genta to become a Buddhist nun. In Episode Six, believing there is still a chance Rin is alive, Mimi, aided by Mishio, displays her resolve to rescue Rin from Apos's sadistic scheme.