A pallid man with soft, damp hands and an obsequious manner, Hallyne is an alchemist of the Guild of Alchemists in King’s Landing, also known as the pyromancers, Hallyne is among the chief Wisdoms (as they title themselves) in the guild and is in regular contact with the royal court. Obsequious and dreamy in his persona, he helps produce the alchemical fluid that the pyromancers call “the substance” and which the rest of the realm calls “wildfire”.
Dotrice is almost certainly a deal older than Hallyne is in the novel.
Hallyne and the pyromancers create a great store of wildfire at Queen Cersei’s behest, but Tyrion comes up with a better plan for its use. Hallyne is present on the walls of the city when Stannis’s fleet is largely destroyed by a ship laden with wildfire being set alight and exploding.