Marin is a maid in the Transylvania manor that you get to have fun with, when rescuing Rinny and getting the shops to open up. After you humiliate her, she turns the tables on you for revenge tests, but they are fun tests!
You'll need Rinny to get to the manor the first time. There will be a NPC by the graveyard blockade that you NEED to talk to, Rinny will make a comment and this allows you to go to the graveyard. It's kind of confusing, and to properly progress saving Transylvania you MUST have Mystic at least 4 (otherwise you get a bad end). Rinny takes the hit for you, but you lose her (temporarily). If you got to a new area, wanted to check something and Rinny stops you, that's where you can now go, you'll meet a mysterious woman named Pixie, who will help you save Rinny and let you enter the manor (until then the manor is blocked off). You need to find a "complete uniform" (not terribly hard) and get introduced to Marin! You'll have to beat her first, but then you get to have fun with her "wink wink nudge nudge" to get some info out of her
No worries about the questions, I like helping! There will be a point where you see a mirror (you'll see a ! and there'll be a warning) and the options are "touch the screen" (bad end) or "resist the urge", but you need Mystic at least 4 to do that. Successfully resist the urge, Rinny will look at the mirror, wondering about it, get caught, and you then need to rescue Rinny. Surrendering to the whispers and the grave hands won't work because they are repeatable enemies. After Rinny gets caught, you'll be able to get to Pixie, who will help you get into the manor. Make sure to wear the piece of uniform that Pixie gives you, otherwise you'll be attacked by the maids and butlers if they see you.