Tsun M! 2 ~Uchi no Kanojo wa Kawaii Pet~

Nemu, his cousin and lover who has run away from home (?).
Since the relationship was approved by both parents, it was natural that she was placed in care.
He made it clear that their relationship would remain platonic until graduation.

However, Nemu finds some SM-related pornographic magazines that a colleague had been keeping for her.

"You won't do it with me... because this is your hobby...?
Because with me, it seems like you can only have normal sex...!?

Nemu began rummaging through her bag and, of all things, handed me a collar.

"Put a collar around my neck... I don't want to wear one, but...
Please put a collar on me like the woman in this book...!!"

So I had no choice but to accept the collar.
I won't give up until you do what I say.

Thus begins the protagonist and Nemu's days with the collar on.