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- “I was advised to keep Catherine in the darkest, deepest hole I could find... Or... Have her removed from the equation entirely. Regrettably, I chose to do neither. Today I shall follow my predecessor's advice… You are hereby ordered to eliminate Dr. Catherine Halsey”— Admiral Serin Osman to Captain Thomas Lasky
Admiral Serin Osman (SN: 39489-72738-SO), born Serin Çelik, formerly SPARTAN-019, is a Human senior officer of the UNSC Navy and current Commander in Chief of the Office of Naval Intelligence (CINCONI).
Serin was born to Pinar Çelik and an unknown father. Her mother was a drug addict and prostitute who left Serin homeless and to pick food from the garbage. The only person who cared for her was her teacher, Alkmini Leandro, who called the police after her disappearance. She was abducted by an ONI agent who offered her a hamburger. Osman was not replaced with a clone as ONI decided no one would notice. Osman failed the augmentation process and was recruited into ONI. Considered a "protege" to Admiral Margaret Parangosky, the Commander in Chief of the Office of Naval Intelligence, she groomed the former Spartan for a potential future as her successor. She is known for her "in-your-face" way of answering people.
She was hand picked by Parangosky to lead a mission to Sanghelios after the war. The mission's facade is diplomatic in nature but in reality the team Kilo-Five are to aid both sides of Sanghelios' coming civil war, the intention being to keep both sides, lead by The Arbiter and religious fanatic Avu Med 'Telcam respectively, fighting for as long as possible, delaying Sangheili reconstruction. For the mission, she was paired with the AI Black Box. At first, she reacted to the eccentric AI with wariness and suspicion, but learned to trust him over time. Serin Osman and her crew had to respond to Venezia to help UNSC Ariadne evacuate. When Venezia refused to help, Ariadne was destroyed when the reactor blew up, she vowed to come back and get revenge. Afterwards she aided in the arrest of Catherine Halsey.
Later when Evan Phillips is caught in the cross fire of the Blooding Years she returned to Sanghelios to save him. When she arrived, the Blooding Years erupted. When UNSC Infinity arrives to aid the Arbiter, she must deftly ensure the crew aboard Infinity do not wipe out the Arbiter's foes. She saves 'Telcam so he may continue his rebellion.
After the destruction of Pious Inquisitor and a report to Admiral Parangosky, Osman decided to check her files prior to the SPARTAN-II Program. After learning the details she decided to go to Cascade and visit her former teacher while the rest of Kilo-Five went on shore leave on Cascade.
As of 2555, Osman had been promoted to Admiral and Commander in Chief of ONI, replacing Admiral Parangosky. During the Second Battle of Requiem, the artifact that had been recovered from Requiem and brought aboard UNSC Infinity began to warp Promethean Knights aboard the ship. During the battle onboard Infinity a single Promethean Knight kidnapped Catherine Halsey. After this occurred Capt. Thomas Lasky contacts Admiral Osman, where she then orders him to eliminate Dr. Catherine Halsey as her predecessor had wanted. Spartan Sarah Palmer agreed to undertake the assassination. However, Lasky dispatched Fireteam Majestic to rescue Halsey, and Palmer was only able to wound Halsey before Jul 'Mdama escaped with her.
During Palmer and Lasky's debriefing after returning to Earth, Osman and other high-ranking military officials questioned Palmer as to whether Lasky attempted to interfere in her objective. Palmer claimed that Lasky sent Majestic as a contingency.
- Admiral Osman has a personal vendetta against Dr. Halsey, after being given Dr. Halsey's personal notebook describing in depth all the actions she undertook in developing the SPARTAN-II program.
- She is the only known SPARTAN ever known to hold a flag rank.
- She is the first SPARTAN washout portrayed in any of the games.
- Halo: Glasslands (First appearance)
- Halo: The Thursday War
- Halo: Mortal Dictata
- Halo 4
- Halo: Escalation
- Halo: Hunters in the Dark
- Halo: Last Light
- Halo: Envoy