Sven Kleinstock was recruited by Magneto to join his second wave of Acolytes. His brother Eric died on his first mission. Sven continued to serve their leader until the fall of Avalon. He then joined Exodus during the Siege of Wundagore.
When Magneto reappeared, the Kleinstocks instead followed Cortez on Genosha. Cortez soon abandoned them and the other rogue Acolytes. His current whereabouts are unknown.
Sven has the ability to merge into one being with his brother, possessing double the size, strength, and toughness that he would as an individual. The merged entity has two torsos, and while in this form the brothers have a merged psyche. He can also generate plasma blasts from his hands.
In the past, the Kleinstocks could create immense waves of flame by igniting molecules of oxygen. This ability seems to have been lost.