Who is Mortis from On a Pale Horse and what is their importance?

Mortis is Death's pale horse. Mortis can appear in different aspects, to help Zane. He can be a horse and talk with Zane through Death's translating stone. As a horse, Mortis can travel to Purgatory and anywhere on Earth. Mortis can fly, though he has no wings. He gives Zane advice and stands by Zane through all his trials. Mortis can also appear in the aspect of a magic car and a magic boat.