Evil Zero

Episode 6
Evil Zero appears as a silhouette in the intro of this episode.

Episode 7
Evil Zero will make his first appearance in the series in the upcoming episode 7, where it is revealed that he slaughtered the men Ceil told Sonic and Shadow to find because they apparently found the Gate of Destruction.

Trivia

 * Evil Zero's design is basically Zero from the Mega Man Zero series, thus he's probably Omega, a villain from said series inhabitating Zero's old body.