Hougetsu Shimamura

Shimamura Hougetsu is a high school student. She met Adachi while skipping class on the second floor of the gym in her school when Adachi accidentally walks in on her one day. The two instantly took a liking for each other and began to hang out together all the time.

In both manga adaptations, it was Shimamura who walked in on Adachi in the gym when they first met instead of the reverse.

Shimamura is a bit of an airhead who loves sleeping and can do so for as long as she wants. Contrary to Adachi, she has grown up on much better terms with her family and as such manages to be much more sociable even if she doesn’t see herself as a particularly kind person. She tends to think about her relationship with Adachi as that of an older sister or a mother. She also enjoys teasing Adachi quite a bit.

(Source: Adachi to Shimamura Wiki)