Thistle and Absol are very close (like, Maxed Out Friendship Levels kind of close) have a lot in common - mainly shared experiences of mobs, torches, and pitchforks. They met in the wilds somewhere after running from some crowd and have just been together ever since. Actually, come to think of it, Absol was never officially captured. But so much time has gone by that it's awkward to bring it up, so they just don't.