We are basically witnessing different routes/endings depending on Keisuke’s choices. The point of the game is to complete those sex challenges without going insane or causing the girls to go insane from all the sexual abuse. In this route he ends up using Byakuya-san as the keyhole to unlock the exit and to pleasure himself and slowly breaking down her sanity with rough sex to the point where she starts showing symptoms of PTSD and having multiple personality disorders. After a bonus challenge of dick sucking, smegma eating, piss drinking, hairjob masturbation and cum swallowing they get a momentary break with dinner and a bedroom all for themselves. Keisuke being the horny bastard can’t stop thinking with his dick and takes the opportunity to have sex with Byakuya-san which started out good but was the last drop of her sanity and she choked him out after he came so much inside her and probably fucked his brains out offscreen. In the end she got pregnant with his babies from all the creampies and he became a crazed sex slave with Nemu supervising the whole thing and likely enjoying riding his cock on occasion.