The Sheet Thief
The Sheet Thief is interested in everything the a character has. Their gold and treasure, sure - it is easier to peel riches from the wastrels who trespass in the dungeon than the fortified residents and gangs who make the dungeon their home. But also the rest of what's written down - their inventory slots, their ability scores, their hair color, their name. After all, the ability to swing a sword, to cast a spell, or haul an extra item around is in much more demand on the Goblin Market.
The Sheet Thief pads around dungeons, looking for adventurers to ambush. They prefer to work in pairs, but can't trust other Sheet Thieves. Starting Thieves will lurk near the lair of known threats who might have an enticing horde, hoping to catch adventurers when they're occupied. But after a few jobs will make themselves a partner out of the traits they filched. These Hodge-Podges will be deployed to chat-up the unaware adventurers while the Thief sneaks-up and unfurls their sheet of cloth. Once close enough, the Thief waves their sheet in-front of the victim, like a third-string stage magician, and captures something from the character's sheet!
Once laden with one - or if very lucky, two! - traits, the Thief turns tail and runs, counting on their Hodge-Podge for a distraction. They'll stash the traits away in their hidey-hole in the dungeon. If an adventurer is diligent and quick, they might find the Thief's home and reclaim their lost bit of self. If they let it pass, they can almost certainly buy it back in the Goblin Market. If they can't make it to market, they can hope the Thief integrates it into their Hodge-Podge, and perhaps pry it from that creature's cobbled-together being on a trip inside the dungeon.
The most cunning adventurers, though, know that tracking the Sheet Thief home is an opportunity. Because if you can pay well enough, it gives you a chance to swap your own attributes with another member of your party.