Before the accident with the prototype Omnitrix, he looked like any other Methanosian. He had red eyes and was slightly taller than a regular Methanosian.
After the accident, his skin turns red and the black parts on him turn into yellow fire.
Infern is extremely self-centred, only caring about himself, and willing to let others die for him to achieve his goals.
He is also very cunning, being able to convince entire species to side with him.
His main motive is to capture Ben's Omnitrix/Ultimatrix MK10, and use it to accelerate his growth enough to turn himself into a Evolved Methanosian.