|This article belongs to the Classic Continuity|
Kwarrel resembles a big grey muscular human with a big mustache and pointed ears and a scar running across his right eye.
Kwarrel was a prisoner of Incarcecon, but besides being a criminal, he had a calm, observant and mentor-like nature to him, as he took Kevin under his wing while he was trapped in his first mutation. While teaching him how to absorb matter, the two bonded, becoming close in a similar 'father-son' type way (due to Kevin not growing up with an effective father figure, this seems likely to be the reason why Kevin was able to bond with Kwarrel easily). During a riot within the prison, Kwarrel and Kevin were caught by Morgg. Kwarrel was willing to give up his life so Kevin could escape Incarcecon and have a future. A selfless gesture of kindness, which Kevin would learn from as he grew up, influencing him as he turned from criminal to hero.
Powers and Abilities
Kwarrel has no known abilities except he appears to be immensely strong as he was able to throw Kevin 11 with little effort. He seems knowledgeable about Osmosians as he teaches Kevin how to absorb matter and free him of his mutation.
Kwarrel appears in flashbacks in ...Nor Iron Bars a Cage. He seemed to have great knowledge about Osmosians and their powers as he was able to teach Kevin how to use his powers better. According to Dwayne McDuffie, "Kwarrel lived a very full life and knew a lot of things". 
He had been digging a secret hole underground for years to get out of the Incarcecon and invited Kevin to come along with him. Morgg was waiting for them, but Kwarrel held him off. He dropped his gun near Kevin, but Kevin was too scared to fire it. Kwarrel urged Kevin to escape, and Kevin ran. Morgg then picks up the gun, shoots Kwarrel and he dies.
Ben 10: Ultimate Alien
- ...Nor Iron Bars a Cage (first appearance; flashback; death)
Ironically, Kwarrel's name sounds like the word quarrel, which means to argue or fight, yet Kwarrel has a calm and laid-back attitude, not to mention occasionally maintaining peace amongst his fellow prisoners.
- It is unknown what crime Kwarrel committed to end up in the Null Void. Dwayne McDuffie hinted that "it was bad".