Arbarax

Arbarax, sometimes known as Arber, was a close personal friend of Cadden Bolt's in the main timeline of Unnamed Ben 10: Alien Universe Sequel Series (main timeline), and an Arburian Pelarota-Human hybrid (one of many things he and Cad shared). He was the same age as Cad, but was assumed obliterated after the destruction of planet Arburia. Unlike many other children that were sent off-world as an excuse to get them away before the Great One destroyed Arburia, Arber was one with his grandmother at the time of the end of that planet's existence.

Trivia

 * In the backstory of Cadden Bolt's life for the Ben 10: Alien Universe timeline, Arbarax isn't mentioned or known to exist as Cad's friend in any way, shape, or form. It is possible that he existed in that timeline, though, and would eventually go on to kill Cad as the two would've become enemies rather than close friends.
 * His existence in the main timeline (UB10AUSS timeline) was confirmed by User:ilikestripes400, writer of the show he first appears in and creator of the Alien Universe series.
 * It is unknown in what form he will appear in the series since he is dead.