Kuro to Kin no Akanai Kagi.

Kanade Katagiri's debilitating anxiety keeps her trapped in her bedroom. She only opens the door for her stepbrother, Ikuto Sonomura. One day, when she makes the bold decision to venture out for a haircut, her social life suddenly blossoms as her new look draws men's attention and boosts her confidence. But these new relationships leave Ikuto feeling that his place in her life is threatened, and Kanade must decide who she wants to cherish most. [Written by MAL Rewrite]