Thief of Heart as a soulkeeper that stores souls of deseased players and uses their low-level flashy powers. Or as a literal soul-stealer/eater who hijacks souls of enemies and assimilates them into their own (with a load of unfortunate side-effects). Or the favourite one - reverse posessions where the "demon" ends up in a horror story where their "host"(Thief of H) slowly overtakes them from the inside. Or persona5esque heart stealing.

Yep, all of these are valid and pretty fun interpretations of the Thief of Heart.