The people who download this app probably already know how to play the game and the different roles of each character.
But for the curious user, or for those who simply forgot what a rare and specific role did, it would be quite useful to include a small rulebook directly into the app.
This rulebook would have to be available from anywhere in the app, maybe from a side-menu, or better, a floating button, so that anyone can pick up the phone at any moment and look at the rules when they have a doubt.