Nothing much is known about Thomas, when he was a human he ran away and soon got bitten by Fred. He didnt want to burn and tried to get the venom out of his body and cut his wrist. Most of the venom made its way out and he began dying of blood loss. Carlisle came across him and brought him to the cullen house, there he turned him. After he became a vampire Jasper and Emmet were always restraining him from Renesmee and Spike Cuther Jr (who were both hybrids, half humans) but managed to escape.