Dr. Adam Lockwood. Or at least he used to be beforehe was turned by Lycans. All because of his friend Michael. Now he awnts vengence on him and Selene. If it weren't for the two of them he wouldn't be this thing he's become.
He was bitten by Raze, Lucian's second in command after the attack on Amelia. Lycans are like vampires in the Underworld verse they pass their memories along through feeding. Finding out Adam was a doctor they thought to continue Lucian's legacy of creating a Hybrid. Adam declined and was thus thrown out of the pack. Ever since he's been on his own. The Lycans want nothing to do with him and he knows very little of what he is.
But ever since his turning he's become a very dark and evil individual. He kills for sport and for hunger and doesn't descriminate on age. Everyone is food to him and he will result to attacking in public. And for that reason the rest of the Lycans want him dead. In the end the only person he has to turn to, is himself.
Info taken from Wikipedia
In the Underworld mythos, Vampires and Lycans are not mystical creatures, but rather the product of a virus. Sixteen centuries ago a man named Alexander Corvinus was the only survivor of a plague that wiped out his village. Somehow the man's body was able to mutate the virus and he became the first true immortal. Later he had three sons, one of which did not inherit his father's immortality and was simply human. The other two were bitten, one by bat and one by wolf, creating the Vampire and Lycan lines. Corvinus' third son also carried the Corvinus Strain, hidden away in his genetic code — an exact duplicate of the original virus, passed along to his human descendants through the centuries. This, according to Singe, is the key to creating a Hybrid.