From Halopedia, the Halo wiki
Clarence was part of Sergeant Lopez's squad when they were sent on a reconnaissance mission on the prison ship Mona Lisa in the Soell system on October 25. Unlike the rest of the Marines, Clarence was fully aware of the mission's true purpose, and was under orders to not permit anybody but the Marines to escape the ship.
When the Marines split up in order to enact the Cole Protocol, Clarence was assigned to Corpsman Benti's group, alongside Gersten, Orlav and Tsardikos. As they were attacked by the Flood in the ship's bilges, Orlav became infected and was promptly shot by Clarence, but not before managing to bite Gersten. After Gersten began to show signs of infection, Clarence would swiftly execute him without hesitation as well.
His status as an ONI operative was revealed to the other Marines moments before the ship's destruction. As the Marines regrouped at the last remaining escape pods, Clarence executed a prisoner named Rimmer, who had made friends with a Sangheili. As Lopez and MacCraw trained their rifles on Clarence, Corpsman Benti forced Clarence into a mob of Flood, becoming assimilated into the Flood. Clarence died when a Shiva warhead destroyed the ship.
Personality and traits
Clarence was known to be quiet and serious, to the extent his silence occasionally disturbed Lopez and the other Marines, who would often talk behind his back, suspecting he had a hidden agenda. He was also an exceptionally skilled combatant, and Sergeant Lopez considered him to be one of her best Marines. He was described by Benti as someone one could depend on and trust. Lopez had noticed how relaxed Clarence always was and mentally noted he was good backup when facing the unknown. It was rumored that he had killed more Covenant over his career than anyone else on the squad. Despite being somewhat ruthless, as he clearly did not regret killing Orlav, Gersten and Rimmer (although he was following orders technically, and Orlav and Gersten were infected by the Flood and he was not to allow anyone other than the Marines to leave the ship), he did have some humanity, saying it wasn't personal when his true nature as an ONI operative was revealed. He also had at least some respect for "Henry", as he referred to him by the nickname and asked him to put down Benti.
List of appearances