Thrax

Early in the series it was never known what Thrax was. Was he alien? Was he a vampire? However, in the story "First Casualties" he learns that his parents were "mongrels", which implies that maybe they were a mix of species or were mutants. 

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 5 years 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".

Maybe he did because 5 years later in "First Casualties", he was in a Veteran's home along with Danny Franks and other apparently dead members of Bad Company, on meds, his memory of his apparent death, repressed.  

His personality was different. Kinder than he used to be, however he became more like the old Thrax as his meds wore off.

Thrax remained in Bad Company and last we saw was continuing on their mission to find proof of the Ararat War Conspiracy. 

It was never revealed whether he was undead or not... or perhaps an immortal being. In the short text story "Ararat" Thrax was killed in an explosion but minutes later is alive and fine. It was revealed to be a dream of Danny's but was it more than just a dream? 

On Bad Company's latest mission he had his left arm blown off, but there was no blood and he was not upset about it, saying that he was lucky he was right-handed. Shades of the Black Knight in Monty Python's Holy Grail.

Another possibility is that Thrax's brain was taken from his body after his death and returned later to his reanimated corpse? (The suggestion is there due to the removal of Mac's brain by Malarkey in "Terrorists").

Or was he simply a manifestation like Golgotha Joe? Or a ghost created by the planet to aid Danny Franks? (Like the ghosts created by the Good Planet in "Bad Company - Kano).