His first assignment is to kill a former scientist of Black Mesa, Dr. Gallhager. However, his mission is harder than he thought because he is a constantly fleeing target, and is accompanied by a seemingly rogue member of the Black Ops.
In the end, he finds Gallhager dead and is captured by HECU troops, only to escape and find his way to Black Mesa, to plant a nuclear bomb.
Instead of being one of the many silent characters of Half-Life and the HLEU, Declan actually voices his thoughts in comic-style cutscenes. He usually speaks about his mission and his job, but can also be a bit sarcastic when he encounters the corpse of his former (and traitor) fellow Black Ops member.
He is set to reappear on Black Ops Source, the Source remake of the original Black Ops.