Originally Posted by StellarRay
Though I could probably look this up.. what are Heart Scales and how many do you need to relearn moves? I've seen the NPC but have no idea how the mechanics work for him. Do I get to pick what move I want to learn? Can I unlearn HM moves this way?
You only need one Heart Scale to learn/relearn a move. You get them from The Underground, and yes, you pick what move.
You can't unlearn HMs this way, though. There's a separate NPC for that.