Caster was a fictional character from the storybook The Kingdom of Elysian, written by Benny Russell. In the story, Caster is a wizard in the Kingdom of Elysian, and secretly the brother of Pollux.
In 2259, as the USS Enterprise concluded a routine survey mission of the Jonisian Nebula, an entity, later named Debra, brought a version of the story to life from Rukiya's mind, in which Chief Engineer Hemmer took on the role of the Wizard Caster. Hemmer's telepathic abilities left him the only other crew member besides Doctor Joseph M'Benga that did not take on the personality of his character. (SNW: "The Elysian Kingdom")