What was with the useless Mamimi character?

Shoegum said:
Yeah, never understood this idea of a character being "useless". In what way are the themes and narrative of one character relevant, but another character's isn't? Sure, there's some focus in the story on a particular character more than another, but a sub-plot is just as much part of the overall narrative.

Mamimi is used in the first episode as Naota's first experience of sexuality, but he doesn't really understand the relationship he's in at all. Mamimi is more interested in his brother, who we never see, and uses Naota as a substitute. Then, in the second episode she sees Canti as a God, giving us some context into what Canti might be. Then in the subsequent episodes, she takes a backseat as Haruko starts to take more of a part in Naota's life. While at first he just learned about sexuality and began to accept his relationship with Mamimi, the more he learns about his own sexuality the more he realises that he doesn't WANT to be with Mamimi, even if he does care for her.

Ultimately, yes, she is just the alternative love interest to Haruko, but we get to see the toll that Haruko is having on Naota's life through his negligence of Mamimi. She makes the viewer even more unsure whether Naota and Haruko should get into a relationship, considering the love that Mamimi has shown for him. The show would've been much less effective if she wasn't there.