I fear I may be drawing them a lot while I work on a project about them, so I thought I'd introduce them properly for those curious.
Aster is a priest, and he worships at the temple of a local river spirit, who preaches pacifism and healing of the sick. During the trial he undergoes to prove himself worthy as a doctor within his priesthood, he runs out of food and nearly succumbs to starvation in the wilderness. When the stranger who appears to ferry his soul away takes pity on him, Aster believes he's been given a second chance, and insists on repaying the debt in any way he can.
Meanwhile the psychopomp Baltair had been tracking a powerful soul, when he finds it within a very weak and very mortal priest. Unable to harm mortals, and unwilling to watch him starve slowly, Baltair extends him mercy. Yet as Aster insists on travelling with him, the lure of such a powerful soul remains-- one which might help him pay his way out of gathering more souls for his demonic master.
[Baltair belongs to my wife, he's just my oc-in-law]