He is in all three games, however, after the first game, he takes a background role with minor involvement.
Throughout the series Julius invented such things as as Power Bunyips, the Techno Rangs, and the Talisman Machine. In the first game he would make Ty new rangs in exchange Golden Cogs he also built the talisman machine which needed 17 thunder eggs to get it to work and teleport you to a boss battle which after winning you would attain another talisman.
In Ty 3, he had been kidnapped by his Uncle Karlos and was forced to work for him. Ty had to rescue him so he could use his technological prowess to fight the Quinkan Invasion. Julius didn't seem to mind working for his Uncle and even mentioned visting him at Christmas.
Role in the series
Julius, as a member of Bush Rescue, helps fight Boss Cass and The Guild of Thunder in The Troupe of Lightning. Since he is one of the many geniuses, others like Tails tend to aid him in his inventions.