Maria-san wa Toumei Shoujo

Maria discovers her invisibility powers when she is 5 years old, but there's a catch—whenever she feels calm or depressed, she starts turning invisible against her will. The only way to turn back or stay visible is to make sure she has heart-throbbing thoughts available at a moment's notice. What better way to keep her heart throbbing than to hide in her handsome senpai's house, naked and invisible, and watch his most private moments? (Source: MU)