From Halopedia, the Halo wiki
Patrick Rimmer was a prisoner held on the prisoner transport Mona Lisa. Rimmer was one of the few humans on board who escaped infection and later was one of the only human beings to make peace with a member of the Covenant: specifically, a Sangheili, whom he named "Henry".
Patrick Rimmer was sentenced to imprisonment on the Mona Lisa when he was found guilty of selling stolen goods, a crime he freely admitted to. He was suspicious of the activity aboard the ship and the strange way prisoners were treated, which was compounded further whenever Covenant prisoners were brought on board. However, he was given more information than most prisoners when a guard named Murray told Rimmer about the Flood, which they both assumed was a biological weapon.
When the Flood escaped containment, Rimmer was nearly killed by an infected prison guard but was rescued when a Sangheili prisoner killed the guard with a cricket bat. The Sangheili did not kill Rimmer but instead freed him from his cell. Rimmer named the alien Henry, and they soon became close friends and comrades despite the fact that they could not understand each others' language. The duo evaded the Flood by locking themselves in a maintenance room.
Rimmer was found by Hospital Corpsman Ngoc Benti, Clarence, and Gersten, the remnants of a small group that went on a reconnaissance mission and was split off from Sergeant Zhao Heng Lopez's main squad. Rimmer told them about the Flood, his experiences on the Mona Lisa, and had to convince them to trust Henry. However, he also told them that Gersten was infected and had to be killed, which Clarence did immediately.
The group, despite Rimmer's warnings, went to the engine room in order to purge the ship's secondary NAV database. There, they killed the pilots (including Burgundy, the pilot who brought the Marines to the Mona Lisa) that the Flood had absorbed into a proto-Gravemind, thus stopping the Flood from learning how to fly the ship. However, Benti was wounded by an infected Marine named Sydney soon after. Rimmer was given an assault rifle to fight the Flood, but was such a poor shot that Benti took his gun and gave it to Henry.
Rimmer, Henry, Benti, and Clarence then fled to the escape pods in order to regroup with the rest of the survivors. Just as they were about to board one, however, Clarence, revealed to be an ONI Section 3 operative who was under orders not to let anybody but the Marines escape, shot Rimmer in the head, ending his life.
Patrick Rimmer was jumpy and quick to panic. However, he balanced this out by talking often, a habit that annoyed many people. He had a habit of affectionately patting Henry, something which made the Sangheili visibly uncomfortable. Despite this, Henry was still fond of Rimmer and was enraged when he was killed.
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