Shiho and Chizuru have been friends since they were children. Chizuru has had a stutter all his life. Later, he also got a social anxiety disorder, which rendered him unable to function on his own. Shiho's always had it all together, so she took on the role of his caretaker when they were still little. This dynamic continued as they grew up, but Shiho wasn't all that happy about it...

...so she took her anger out on Chizuru.

[Translated from DLsite]