Devlin Levin

Devlin Levin Tennyson is Kevin's (biological) son, Kenny's best friend, and later, Ben's adoptive son. Kevin named Devlin after Kevin's dead father, Devin. When they first met, Devlin tricked Kenny into leading him to a Null Void Projector which he used to free his father.

Appearance
Devlin bears a striking resemblance to his father when he was his age. He is tall, has long black hair pulled back in a straight ponytail, dark blue eyes and pale skin. He wears a dark gray short sleeved hoodie, a black shirt, black pants, black and white shoes and a bandage on his right leg.

Powers and Abilities
Devlin can change from human to a mutant alien form at will. His mutant form is similar to the one his father had at his same age, a conglomerate of Ben's first 10 aliens, but he did not inherit his dads ability to absorb energy, (confirmed in Ben 10 pop-ups for Ken 10) which allowed him to keep his sanity and not turn out like his father.

Family
It was revealed that Devlin is the son of Kevin and a girl from a Saturn colony, which he probably had a short relationship with. He is later adopted by Ben, becoming his adoptive son and Ken's adoptive brother.

Trivia

 * Apart from having the same mutant form as his father did at his age, Devlin shared many aspects with Kevin: He met Kenny in the same way Ben met Kevin, after Kenny went off after getting upset with Ben much like how Ben stormed off after an argument with Grandpa Max. Unlike Kevin however, Devlin genuinely liked his friendship with Kenny. Furthermore, he does not have the sociopath tendencies his father had at his age and, in the end, was approached with the same offer to join the team that Ben offered Kevin. Unlike his father, who turned the offer down (at least until Alien Force), Devlin gladly accepted it and joined the Tennyson family.
 * It needs to be noted that Ken 10 isn't a "canon" episode, that it shows only one of the possible futures and not the one the series is currently leading to. It may also be interpreted as a possible future outcome that had been averted by the choices and the help Kevin needs to use his powers properly.
 * Like Kenny Tennyson because of different choices he doesent exist in the Alien Force and Ultimate Alien timeline and he only lived in an alternate timeline.
 * It's possible Devlin was named after Kevin's father Devin.