Rex Vulcan is the evolution of Premo Vulcan. His features are upgraded versions of Premo Vulcan's. His knuckles are shielded from his spinning rings. His bull horns go up instead of across. His eyes are still covered by the red specs, but his real eye color is brown and wild. He also has shoulder pads opposed to his pre-evolved version. His body is now bigger and he also not only runs while battling but slides. His knuckles also had two new features when his Fusion Ability activates, they get drills on them instead of spinning when he was Premo Vulcan and before the fusion ability the knuckles get horns on them for piercing. His knuckles also turn into cannons when he does not fire them. He still has his free will (because he and Elico didn't struggle when Chaos Ability X was used on them), even though he was evolved due to Chaos Ability X.
In Avenging Spectra, he faced Farbros and has trouble but looked like he had the upper hand when Blast Elico changed his attribute to Subterra. In this battle it also shows that he still had his own will as he could speak and he protected Gus when Farbros tries to kill him. After Elico and Hexados were killed he tried to protect Gus because he was still loyal to him. He was blown away by Farbros along with his master, Gus.
At the end of An Heir To Spare, it is shown Gus is alive in a cell and Rex Vulcan was too.
In Ultimate Weapon he helped Gus escaped from his prison cell and battled King Zenoheld and Farbros for a second time. During the battle, he was able to beat-up Farbros, and, together with Helios MK2, outmatched even Assail Farbros, thus making him the third Bakugan (after Dryoid and Helios MK2) that won against Farbros and almost defeated Assail Farbros.