How far into the game are you? Have you had a “Talk About Sex” conversation with the Best Friend(thats the technical term for what Guillermero is)?
My next paragraph is somewhat spoiler heavy for how you start the process of growing closer with The Best Friend, but not that bad for the two ways you get in really deep.
If your doing what I think you are its actually simple, go to the Main Hall at any time between 6:00 PM and 10:59 PM, I strongly recommend going to their room every night, there are some chance based things you need to not happen and you need to have 2 specific events happen within the “Talk About Sex”