Mimina is a 19-year-old girl who looks more like a 10-year-old due to an illness.
First shown in episode one, Mimina is yet another victim of Akiharu's grope fest while he is fleeing from Selnia. While she has literally no chest to speak of, Akiharu manages to grope something while she is squirming around in his grip (he was trying to keep her quiet so Selnia wouldn't find him). We then see no more of her until Akiharu is assigned to spend time with her by Tomomi for his mid term final.
At the start of the assignment, Mimina is very resentful of Akiharu's simple presence. Repeatedly ordering him to leave her alone. Although through sheer pigheadedness Akiharu starts to win her trust. Eventually helping her draw a kitten that is positioned in his lap. However, the kitten is scared away by a slight movement on Akiharu's part, prompting Mimina to run off. After a good merry chase Akiharu finds her, then offers to help her find the cat again like one would act towards a little sister. While searching for the cat, with Mimina riding on his shoulders, they happen across some upper ed art students that are outraged at how casually Akiharu is treating Mimina. I believe the verbatim line is, "it is an artists duty to draw for everyone." To which Akiharu responds that Mimina doesn't have to draw anything unless she wants to. When the two upper ed girls start to get pissed off at him Akiharu threatens them with his reputation as a harasser to get them to leave. Shortly afterwards, they find the cat and Mimina finishes the drawing, including Akiharu.
Interestingly enough, the whole time Tomomi was spying on the pair, as was Selnia.