Ok, so instead of picking up some coins to unlock Rin you actully have to answer 10 question in a row, which I assume tests your knowledge of the franchise (or in the very least, if you have access to google). Not bad of an idea to be honest but . . . well, I guess I dont need to tell you what the problem with that is.
So yeah, I just figured the answers out by trial and error. I will try my best to make clear which answer to pick for what question. Hope my system is not to complicatet.

First go int the hall on the left side of the ub area and enter the "Heros"-Room. Their will be a slot machine where a shrine should be. It can ask you 1 of 10 question wihich you will have to answer right, so the next on pops up. You wi have to answer all 10 questions to unlock Rin.
So, I will try my best to describe the difference questions:

1.) 3 Options, one has the word "Another Story" in it ---> Pick the one with "Another Story"
2.) 3 Options, medium length with the longest in the center ---> left answer
3.) 3 Options, left answer has 5 charakters, center one 4 an the right one 4 ---> middel answer
4.) 3 Options, the left answer is way longer then the other two ---> left answer
5.) 3 Options, a long answer followed by a short one and a long one on the right side (the last symbol in the right answer kinda looks like a 7) ---> middel answer
6.) 4 Options, length of the answers: 5-2-3-2 ---> secound answer
7.) dunno ho many Options but one answer includes a "50" ---> pick the fourth option (might be the
last one, i dunno)
8.) 5 Options which all consist of 4 charakters ---> pick the fourth answer
9.) 5 Options where oneincludes a bracket ---> choose the third one
10.) 5 Options which consist of the following number of symboles 5-2-3-4-2 ---> pick the last one


That should do the trick.
Ones the game gets a english translation I will delete this guide. But since you are kinda looked out of this content if you cant read any of this stuff, I though this whould make sense.