Amaia from Princess of Swords, a story created by my mate.

Amaia eats ghosts. She’s a living, breathing, physical person, but she’s capable of astral projecting and interacting with the spirit world. Devouring souls doesn’t feed her physically (she still has to eat normal food, too) but gives her an enormous rush and potentially allows her to retain some of the memories of those eaten.

The souls she eats aren’t destroyed or permanently absorbed, but rather are “freed” in a sense. Spirits remain in the living world because they are burdened with mental energy, often in the form of unresolved issues or mental anguish (due to an untimely death.) They will continue to exist as ghosts until all that energy is used up, at which point they will pass on to the next life (which is as much a mystery to Amaia as it is to us.) In fact, once a spirit is lingering as a ghost, they can’t move on until all that energy is used up, even if they want to, which can take centuries depending on the circumstances.

When Amaia “digests” a soul, she is consuming all their remaining mental energy. Without that energy, the soul is no longer anchored to this world and is forced to move on. The process is painless for the ghost being eaten. What Amaia does could be seen as a boon.

Of course, most ghosts, even those who do want to move on, don’t see it that way (having your soul be eaten is pretty damn frightening,) so Amaia rarely even tries to explain the situation. She draws ghosts to her with her strong living soul, which, once projected, acts as a natural beacon to other spirits. Once she’s close, she can create an aura that will overpower weaker souls, making them disoriented and compliant. This effect doesn’t last long, and often wears off before Amaia is done eating, but by that point it’s usually too late for them to do anything about it.

If you're interested in reading more about her, check out Krys' Tumblr blog.

Edit: Krys pointed out that I didn't have a link to the original story, so here it is! We're still discussing ideas, so not all the minor details here may be canon. Some things may change for the continuation.