Marin

Marin can be found on Manor's second floor (to the right from Master bedroom), during or after Manor quest.

Interrogating the maid
During Manor quest, Marin can be interrogated to learn where prisoners are kept. When approached, the option interrogate her appears. To interrogate her, she must be defeated first; this battle goes similarly to normal battle with the maid, but she seems to have higher stats. Losing to her would result in Bad End, similarly with the any other servant.

Revenge
Marin may be encountered again after you get your limbs detached by Edith, if you visit her room and choose to be put on the bed. She would try to make her own revenge on you, by starting molesting you in turn... which would open her for attack from Rinny, making her your hostage yet again. You can ask Rinny to pay her back, or just leave her as is.

Note what this even wouldn't trigger if she was not interrogated before. If you choose to hide under the bed, you would skip this even regardless of whether she was interrogated or not. This wouldn't affect post-quest interactions.

Marin's challenges
Marin can be encountered again after finishing Manor quest. If she was never interrogated, she wouldn't recognize you. Fortunately, special trigger is presented right in her room, which would "rewrite the history", allowing you to continue as if she was interrogated.

After she recognizes you, she would offer you to pass certain "challenges" for her. This certainly wouldn't be a trap, would it?..

First challenge
See also: Marin's first challenge

If Lust Doll visits her after Manor quest, they would learn that Marin still has grudges against them. Whether they apologize or not, Marin would ask you to do something for her before she would forgive you. That challenge requires you to wear specific underwear while giving her massage. The catch is what underwear is cursed, and would start molesting you the moment you put it on.

Acting nice and trying to please her would pass the challenge, while acting rude (or outright attacking her) would fail it. Regardless of whether challenge was passed or not, she would offer to sell underwear to you. The price depends on how she was satisfied, with lowest possible price being $800. If player was rude, the price can be quite high.

Second challenge
See also: Second Marin's challenge

The second challenge is available as soon as the first one is finished, and requires wearing the servant uniform (similarly to Manor quest) all the time.

This time catch is what you should work as a servant, while wearing different magic underwear, which would allow Marin to molest you from the distance while you work. When, with Rinny's help, you pass it, you can force Marin herself to do the same challenge, with you being the one to molest her. If you know the dick-growing spell, and have at least 6 Speech, you can insist on Marin growing one for duration of the trial.

Marin would never accept defeat, and so, the second challenge can be repeated whenever you want.