It was never known what Thrax was. Was he alien? Was he a vampire? One thing for sure, he was certainly a psycho and one of the more memorable Bad Company characters. He would call people "Turnipheads" and threaten to "Slash apples". He had little concern for any of his comrades, but seemed to have a thing going with Shrike.
Thrax was kind of like the 2nd in command and a valuable member of Bad Company. He was often questioning Kano and accusing him of "going soft". He was even determined to kill Kano or at least see him die, but later on, when he returns as a ghost in Bad Company - Kano, we discover that although he claims to hate Kano, he does have a begrudging respect for him.
Thrax was killed in the final battle on Ararat, but that was not the last we saw of him as he returned later in Bad Company - Kano as a ghost to aid Kano, along with some other deceased members of Bad Company. He claimed that he had "Made it out of Ararat alive".