While in battle and other occasions you should be able to hold anywhere on the screen to speed up the enemy’s turn or speed through dialogue. And the crystal collector shows up early on in chapter 1 I think. He sells mainly cheats like extra healing potions. Soon you’ll notice that you need bronze, silver or gold ‘proofs’ to replay certain levels. He’s got your back if you need extra of those. But if you play like me (sounds like you do) you’ll collect plenty just by playing. Also, if you’re running short of play time every now and then he sells something to let you collect all chests in a level immediately. I found that there’s so many dungeons in the game that I used these every now and then, they really help for us ‘completionists’ but aren’t necessary. Hope that helps
