Mr. E

Mr. E is a General of the Sons of Garmadon. Little is known of Mr. E; he wears a black biker outfit, red gloves, and a black helmet with a red visor. His eyes appear to be glowing red, because he is wearing a mask. He is very mysterious and he almost never speaks. He also has the letter "E" on a badge on his chest.

The Mask of Deception
Mr. E led a group of Sons of Garmadon to the Borg Tower in order to steal the Oni Mask of Vengeance. His attempt was succesful, however a member of the S.O.G. activated the alarm and Lloyd came to stop them. Mr. E was force to abandoned his team and fled the scene. He then returned to his bike, but Lloyd kept chasing him with his Ninja Nightcraweler. He found a way to jump over him and land onto a boat which he used to escape.

The Jade Princess
Mr. E, along with Ultra Violet and several other Sons of Garmadon members, assault the Royal Palace with the goal of stealing the Oni Mask of Deception. While the others bikers fend off the Royal Guards, Mr. E drives his bike through the palace to the location of the Oni Mask of Deception's display case. However, he is cornered by Cole, Kai, Nya, and Jay.

Anticipating this, Mr. E reveals the Oni Mask of Vengeance to them before putting it on, causing him to gain two additional arms and swords. Combating the Ninja, he is successful in deflecting their attacks and powers, even utilizing Jay's Lightning to knock back the Ninja. Smashing the display case, Mr. E claims the Oni Mask of Deception. However, Kai summons a fireball and shoots it at him, destroying the Oni Mask of Deception, much to Mr. E's anger.

Fortunately for Mr. E, the mask that was destroyed was a fake; the real one would be recovered by Ultra Violet after she chases down Lloyd and Harumi.

Ninjago.com Description
''Mr. E is a member of the Sons of Garmadon, known for the fact that he hides his face behind a mask and never speaks. A master swordsman, he is Zane’s newest nemesis.''

Spoiler Notes

 * Dan and Kevin Hageman may have confirmed the identity of Mr. E to be the Overlord in a tweet where they responded to a tweet of Mr. E's eyes with a picture of the Digital Overlord's eyes.
 * Further proof to back up this claim is that LEGO.com's description of Mr. E is that he is "Zane's newest nemesis." The reason for this could possibly be because Zane was the last person to defeat the Overlord.
 * However, this theory doesn't make much sense, considering that the Overlord doesn't have any reason to resurrect Garmadon.