Marin's second challenge

This quest is only available if female partners are enabled, and Marin was interrogated during Manor quest; if she wasn't, and first challenge was skipped by buying underwear from Dalvie, there's a switch which overwrites her state to assume that she was interrogated.

Preparations
Leave Transylvania completely, then return again.

By that time, Marin would prepare the new challenge for you. This time, you would require the full set of Servant Uniform first. If you still have one from Manor quest, just put it on, otherwise, you can buy new one on the first floor of Manor.

The trial
Marin would require you to do some chores while wearing the servant uniform... and another special underwear she obtained just for you. The catch is that she would be able to molest you without leaving her room.

No matter how hard you try, you would fail the moment Pixie would approach you. However, if you bring Rinny with you, she would distract Pixie for just long enough for your orgasm to go unnoticed. Even then, you have to pass the Resilience check to hold until that moment, with 25+(3*Res)% chance of success. If you fail the quest, you can retry it at any moment.

Once you pass the test, Marin would be forced to attend the same challenge herself, allowing you to molest her.

Because Marin would never admit defeat, she would require a rematch, essentially resetting this quest. You can rematch her as many times as you want.