Thomas Aries is a human, born on the Mars colony. He was the warden of the Acheron Maximum Security Prison - until the Acheron Infestation and the prison (and planet) was overrun by xenomorphs.
Aries survived when the prison was overrun and managed to evacuate...barely. He suffered terrible injuries, and doctors did not believe he would survive.
But he did. Because of his service as a warden, he was compenstated with cybernetic implants so he could walk and function again. Very little could be done about the acid burns on one half of his face and a fair portion of his body, however.
The scars and nightmares of the Acheron Infestation made it almost impossible for Aries to re-adjust to civilian life back on Mars. When the opportunity arose, Aries reapplied as warden of the now Orbital Acheron Prison, where he currently resides.
According to his peers, since the Acheron Infestation, Aries has become a mistrusting individual with a particularly bad temper.