Ren Krawler

A Gundalian disguised as a human, he was sent by Emperor Barodius to find the strongest brawlers to help defeat Neathia and to find the DNA code. Ren comes from the underground world of Gundalia, where he was kept by Emperor Barodius' family because he possesses a dark Bakugan with a very powerful ability. His Guardian Bakuan is Darkus Linehalt and his Battle Gear is Boomix. Later, due by the Dan, Ren turned on Barodius because Dan made Ren remember it was Barodius that banished his people and he finally chooses a side. Like worse in episode 24 Linehalt released his forbidden power.

But Fabia doesn't believe that Ren is a changed person so, in Episode 25: Fabia is going to battle Ren and going to find out the truth. (Source: Bakuganwiki)