Colonel Cuckoo was an immortal and British soldier who has served in the military since the Napoleonic Wars. In 2009 he was serving in the war in Q'Mar. Cuckoo met Orlando, who was also serving in Q’Mar, and learned to his shock that his fellow immortal admitted to committing a massacre that touted Orlando for having surviving it. Though Cuckoo doesn't judge Orlando's actions, he understands the temporary madness Orlando describes as a regrettable but unavoidable consequence of living forever.
Colonel Cuckoo is based on the protagonist from the Gerald Kersh story Whatever Happened to Corporal Cuckoo? (1953) in which the titular character is rendered immortal following the battle of Turin in 1536.