Aki is the younger of Boku's twin sisters. Boku refers to her as the "cheeky", selfish imouto.
Aki was originally in love with Boku's now-deceased older brother (referred to as Ichi-niichan), whom she viewed as a very responsible and incredible person. Thus, she in part wants Boku to be like him, but decides to make fun of him for being an otaku and a ronin. She's not as merciful as An is, which often puts her and Boku at odds with each other. They do get along through their common interested in anime, as she herself is a fan (and a cosplayer). Aki is also a lot more open when it comes to talking about sexually themed things like eroge, much to our protagonist and An's dismay.
Unlike the rest of the cast, Aki is very detached from the events of the game, basically doing her own thing while studying to be a voice actress. Just like An, she moves in with Boku to make sure things are alright with our protagonist, but also because there's a nearby acting school that she wants to attend.
A point of interest is that due to Aki's low level of interaction with the other characters, it is through her work that the protagonist develops feelings for her. Her talent as a voice actress shines in the story, since she is contracted to be a VA for "Ai kawarazu na boku". Much to Aki's chagrin, Boku prefers to see her act as the innocent and cute imouto (which is nowhere near what her real personality is).