Aya has a tomboyish personality, which likely developed after years of sticking up for Suiren. She doesn't appear to have a problem with this in the least. She does appear to feel lonely when Suiren and Kawasumi begin dating, feeling left out. Usually carefree, or just plain disinterested, Aya only truly shows any interest if it has to do with Suiren. She is very protective of her because she understands how Suiren has been treated since she was young, due to her looks. She seems to be much more mature than the other characters in the manga's cast and seems to be the least affected/invested emotionally in Kawasumi and Suiren's relationship.

Here's her voice in jpn: https://youtu.be/sZaIc5kL3io?t=86