I think the relationship can be divided into 3 phases: the Cute phase, the Ugly phase and the 'Cute' phase:

1. Initial Cute-Peaceful Phase. Cute high-school romance, or it's what the girl thinks until she finds out the extreme masochistic tendency of the guy. The phase is about 2 persons.

2. Intermediate Tension-Anger-Revenge Phase: The girl is appalled by her 'pervert' boyfriend, so she left him, but deep inside she can't leave the relationship. She decides to take revenge by torturing the guy with a new guy. The thing is that no one forces her to do so. She could simply leave the guy or force him to go to a doctor or so. But she uses her power to create all kinds of extreme sadistic situations because deep inside she enjoys it. She says to herself that a new guy and a new love can solve the problem, but it doesn't work. She gradually notices the horrifying truth: she is sadistic. As the last resort, she thinks of the death of herself and then of the guy. But when the guy follows her order, she realizes that it's not what she wants. In this phase, 3 persons are involved.

3. Final 'Cute'-Peaceful Phase: Finally, the girl comes to peace with her sadistic tendency, and with that, also comes to peace with her boyfriend. Therefore, we have a cute-peaceful atmosphere again. The difference is that the relationship is sadistic but without any feeling of anger or revenge. Now we see them sitting beside each other and the sadistic behaviour is reduced to asking to get a drink or caressing the arm, so from a distance, they look like normal lovers. They might introduce some guy into their relationship, but just occasionally and for fun.