This doujin has some sort of happy ending if someone is wondering.
The plot in short:
-Mash loses her ability to fight, but she wants to be useful to Ritsuka -She goes to Kiara who is "therapist", and when Mash closes her eyes, Kiara shows her visions of being gangbanged -Kiara says that Mash can be useful to him with her body -One month later Ritsuka wants to spend time with Mash, but she says she have her counselling session -Ritsuka spots her being gangbanged and wants to save her but he is being held up -Mash is happy that he can see her and explaind that she is helping him by managing desires of men in chaldea since this is closed environment -After gangbang she precises that Kiara told her that Ritsuka has NTR kink and she wanted to make a good show for him to be useful to him as slut -Then Mash gives him handjob and promises to handle his ejaculations everyday -Then...Ritsuka opens his eyes and it seems now he had "vision" in Kiara's "therapy" just as Mash had before