Envy aka desiring to be someone else, to have all their powers and positive qualities, desire to be them and to "replace" yourself with them because "the self" on it's own is already worthless and can never be good enough as one of potential "lows" of Thief of Heart. Oh, and an identity crisis after stealing too much Heart.

That’s an interesting way to see a Thief of Heart at their lowest, yeah. Hmm! Paired with the literal power of actually switching bodies with that they’re trying to emulate, it could make for an interesting story.