The character that visits you in the inn is determined by who has the highest affection when you get there. So, just make sure you have Erwin's affection higher than everyone else's. :) You can right click to open the menu at any time to check affection, and you can use a scroll wheel on a mouse (or PGup) to roll text and choices back.

I hope that helps! If you have anymore questions, feel free to ask!

(oh yeah, you also have to pick the right choice when he comes into your room. I can tell you which that is, if you want. Or you can roll back until you get it. It might not be what you think it is!)