Hugh  

The character you're referring to is known as Hugh, who serves as the main antagonist in series 1 and a significant adversary in series 2. He leads the group of three Zymot Usurpers and wears a mask until Amil Gaoul rejects him. Despite his strict demeanor, he becomes unhinged following this rejection. Prior to being defeated by Sara, Hugh was the owner of Pronimo, the Key Spirit that was later stolen by her.