Moss seemed to be mentally deficient, nearly mindless. He did whatever Osmere asked of him without question, and Osmere noted that he had had his tongue removed prior to becoming a servant but that he likely wouldn't have much to say if he could talk.
Moss is huge and physically imposing.
Moss was sent with Desmond to intimidate or kill Queen Helen. He participated in several fights involving Hector and his allies, but neither they nor his side ever achieved a decisive victory while he was present.
He was later sent to accompany Geoffrey on a trip to hunt down Colt, but along they way he and Osmere met and recognized Damian, who killed them to keep his secret. Before doing so he asked them whether they were from the Morgunov or Dozer faction, although it is unknown what importance this information had to him and whether or not he would have spared them if they had answered Dozer.
Moss had the power of chlorine transfiguration. The only thing he was ever shown doing with it was vomiting hydrochloric acid as an attack, and it is unknown if he knew any other techniques.