Aidan is Death. The (or maybe A) Grim Reaper. A physical manifestation of the inevitable end of all things, tasked with guiding the souls of the dead to the afterlife by helping them let go of their worldly ties. The bulk of her job is kind of like sales, convincing those that have died that they need to let go of their worldly dies and allow themselves to cross over into the unknown.
Spirits who refuse to let go are doomed to eventually go mad and become monsters. Aidan's other job is to destroy those. She likes it a bit more than the talking to people part.