Kevin 11 appeared, officially as a cameo, in Avatar Aang Vs. Ben Tennyson. He was voiced by Molemanninethousand. Kevin appears following Zuko's initial verse and his boast of representing a great redemption story, interrupting the line "...so don't you dare even mention–" (which was obviously going to refer to Kevin) and attempts to begin rapping as Zuko's counterpart for Ben Tennyson's side, only to be immediately killed by the young Fire Lord.
Information on the Cameo
Kevin Levin is a character in the Ben 10 franchise, introduced in the original series as a sociopathic, delinquent thug with the ability to absorb and copy energy and powers. In his debut appearance, he appears as the main antagonist and attempts to cause a massively fatal train–to–train collision to rob the vehicles' contents, and escapes having successfully obtained a version of Ben's Heatblast powers. In later episodes, he further mutates into a giant monster amalgamating features of all ten Omnitrix aliens, and is consistently portrayed throughout the original series as an unrepentant villain.
Starting in Ben 10: Alien Force, however, Kevin (admittedly years later in–story) makes an unexpected turnaround and becomes one of the heroes. He is currently Gwen Tennyson's love interest.
Sup Bitches, they call me Kevin 11,
And I'm about to send you to the opposite of Heaven,
Cause I got a– *Wilhelm Scream*
Like other aspects of the later Ben 10 series, Kevin's completely non–foreshadowed redemption has been considered implausible and indicative of the series "Jumping the Shark", even though he has now been a hero longer than he was a villain. This controversial U–turn in character development is comparable to that of Vegeta from Dragon Ball Z, hence the line delivered by Zuko immediately after killing Kevin, whose "out–of–nowhere" redemption stands in heavy contrast to that of Zuko, who is portrayed with sympathetic nuance from the start and goes from villain to hero in a gradual, believable manner in a story arc that runs throughout most of the Avatar series' course.