Clayf in disguise
Clayf was a Bakugan and one of the six Legendary Soldiers of Vestroia. He was the soldier of Subterra.
Clayf in his true form
He, alongside his comrades, appeared to the Brawlers in the Doom Dimension but disapproved of asking them for help. He used an imitation of Julie's elder sister, Daisy, who would always beat her at everything, to battle her in a fake, abandoned ghost town. Clayf eventually revealed his true form. Julie learned to not compare herself to her older sister and Gorem, her Guardian Bakugan, evolved into Hammer Gorem thus passing Clayf's test and defeating him.
Clayf, having been defeated by King Zenoheld, gives his Subterra energy to Thunder Wilda. This, in turn, causes Thunder Wilda to evolve into Magma Wilda.
And he is voiced by Phil LaMarr in the English dub and reprised by Taro Yamaguchi in the Japanese dub.