It's technically a 4 AM magic system, but basically they use their own souls until lichdom, in which case they are technically remotely piloting their corpse that uses life energy essence shit it absorbs at very small increments ambiently. Unless they go the soul steal route.

Additionally, I'd limit it to they can't cast magic in general without mana. Now as to how strong a guardian is... honestly that was something I hadn't considered. I would say maybe that unless you undo the ritual binding a corpse to its owners will, then that's how you remove it entirely aside from just destroying it? I'm open to suggestions and changes though, since I did think of this at like, 4 in the morning a month or so back.