Kei Nagase was a pilot of the mercenary unit Scarface Squadron during the Usean coup d'état. She flew alongside Phoenix in certain missions that Phoenix chose to fly with her. Following the war, she retired from military service and became an airliner pilot for Air Ixiom.
Usean coup d'état
Assigned to assist Phoenix as his Number Two, she joined Scarface Squadron in the hopes of putting an end to the war and to her home country's suffering inflicted by the coup d'état.
Following the war's end, Nagase retired from military service. She then joined Air Ixiom as an airliner pilot, a profession she always dreamed of having. On March 14, 2005 during the Continental War she was the co-pilot on Air Ixiom Flight 701, one of two flights with various Stonehenge engineers on board looking to defect to ISAF. Erusea learned of the defection and launched multiple fighters to shoot down the civilian airliners. One fighter damaged Flight 701, injuring the captain and forcing Nagase to take command of the aircraft. 
ISAF sent Mobius 1 to protect the airliners and lead them to safety. Due to cabin pressure complications from being shot, Flight 701 had to maintain an altitude of 6000 feet whereas Flight 702 remained at 23,000 feet. Mobius 1 successfully shot down all Erusean fighters and the two flights landed safely at an ISAF member's airfield. All of the passengers and crew were safe and deemed to be in good health despite the battle, including 701's captain and Nagase.
Personality and characteristics
While Nagase lacks both physical strength and experience, she makes up for it with flying intuition that rivals veteran pilots. She is a realist who prioritizes the quickest and most effective tactics during missions, but can also come off as cold and stinging as her codename. She leaves her feminine qualities on the ground, often catching her male counterparts off-guard in the air.
- Ace Combat 3: Electrosphere - Mission & World View Guide Book, page 182.
- Issue 189, Computer and Video Games. Archive.org. Originally published in August 1997. Retrieved on 3 October 2017.
- Character biographies. Ace Combat: Assault Horizon Legacy.
- Ace Combat 04: Shattered Skies - Official Guide Book. Page 16.
- Ace Combat 04: Shattered Skies - Escort