From Halopedia, the Halo wiki
Lieutenant Commander Jilan al-Cygni was an officer and operative of ONI Section Three at the time of humanity's first contact with the Covenant on planet Harvest. She assumed the role of ranking military officer on Harvest during the initial Covenant invasion of the planet and subsequent evacuation of the human population. She served as commanding officer of the Staff Sergeants of the planet's fledgling colonial militia (including Avery J. Johnson and Nolan Byrne).
Nothing is known about Jilan al-Cygni's career prior to her actions on Harvest, although it was once observed by the AI Sif that, judging from al-Cygni's subtle accent, she was likely raised on the planet New Jerusalem in the Cygnus system.
In response to reports of attacks on DCS freighters in space nearby Harvest, ONI deployed al-Cygni undercover to the colony world to investigate and neutralize suspected Insurrectionist activity. Al-Cygni posed as a DCS middle-manager and arrived with the pretense of conducting an audit with the AI Sif for negligent maintenance of shipping vessels. After making contact with Governor Nils Thune, she inducted Staff Sergeants Avery J. Johnson and Nolan Byrne into a space operation to lure Insurrectionists to a decoy freighter where they would be neutralized. To their immense shock, the fire-team (comprised of Johnson and Byrne) encountered not Insurrectionists, but hostile alien creatures. Al-Cygni authorized Johnson and Byrne to eliminate them before destroying the alien vessel with an Archer missile from her sloop.
Later, after making contact with the Brute investigation team arriving with a formerly decommissioned cruiser, she was among the first humans to meet an official Covenant delegation. The negotiations soured, however, and the group was forced to escape. In the week following, she coordinated a pending mass evacuation maneuver. She removed a visibly upset Governor Nils Thune from office by paralyzing him with tactical training rounds, and assumed command of the UNSC presence on Harvest, consisting of a mere three Marines and a platoon of Colonial Militiamen.
After many risky and difficult plans, in one of which her second in command volunteered to create a diversion at the cost of his own life, al-Cygni successfully escaped with Johnson, Byrne, and a handful of to-be-Marines and civilians. During the escape, she and Avery Johnson had sexual intercourse. It is unknown if the two remained together for any period of time or on whether they had even seen each other in the years since.
It may be possible that Johnson and Jilan al-Cygni were married, as Johnson will occasionally make remarks about his supposed "mother-in-law" during Halo 2. Whether it was just Johnson's typical sense of humor, or actually meaningful in any way is unknown.
List of appearances