When we’re first introduced to Sakaki Yumiko in the common route of The Fruit of Grisaia, it’s clear that she is both troubled and likely to be trouble for protagonist Yuuji.
When Yuuji first attempts to introduce himself to her, she initially tries her best to ignore him, and subsequently goes to slap him when he persists. This eventually escalates into her lashing out at him with a box cutter whenever she sees him, a fact which Yuuji’s fellow students just shrug off as being “something she does”, because they’ve all been through it too.
Yuuji, being a certified badass, shrugs off Yumiko’s attacks without injury easily, and eventually they stop altogether, though she still proves herself to be a prickly individual who is generally unwilling to interact with others.
Except, of course, it’s not that simple.