The BS Situation of Tougetsu Umidori, Vol. 4 (light novel)

"One day, Tougetsu Umidori suddenly receives a mysterious request from Nara Yoshino, a classmate of hers she is rather close with. Little does she know, however, that this is only the beginning of a series of bizarre events. Soon after, an enigmatic girl in a cat hoodie shows up at Umidori's house, calling herself "BS."

Thanks to her, all manner of strange things happen, including toilet borrowing, betrayal, threats, a grapple for freedom, and finally a desperate plea for mercy. Once everything is over, this strange new girl asks: "Hey, will you kill the lies with me?"