Steinbeck is a mutant arms dealer who, in his dealings, killed a mutant named Prudence Leighton. Before she died she psi-jumped into his mind. After being double-crossed by Mystique, the inseperable pair exploded, insane and unable to control their powers.
He might be a hermaphrodite since Prudence Leighton possessed him despite the fact she only can possess women.
Steinbeck possesses a pyrokinetic breath allowing him to spew pyrokinetic flames from his throat cavity.