The Mysterious X
Not much is known about X, besides the name, though it seems that he may or may not be Gary’s long lost brother that ran away from home when Pops wouldn’t let him race anymore but he won’t tell Gary because that’s a secret.
The Dark Wizard Fresno
This one seems to be pretty self-explanatory. Rezo was the main villain in slayers, so is Fresno the main villain in The Mystical Adventures of Gary Lightblade. Though it should be noted that Fresno is a dark wizard, not a red priest.
Strangely enough, Nagha is completely unchanged from Slayers to The Mystical Adventures of Gary Lightblade. Apparently Sadam thought, given her proportions, Nagha was an American character anyway. In fact they plan on trying to put her in as much as possible, taking stuff from the OAVs and movies and editing it into the TV show.