n order to play as Crystal, one must first get the Family Jewels [ihrb.wikia.com]achievement. Compared against the other two characters unlocked by collectibles, Crystal's is the easiest, due to hers being on ground level in a pretty visible location. Chances are the player will destroy her in-world collectible without even seeing it.

Character wiki page - http://ihrb.wikia.com/wiki/Crystal