Kei Domon serves as a cadet aboard the space vessel Blue Noah. She is the daughter of Commander Eiji Domon, a leader within their shared military structure. Her background includes direct involvement in the critical mission at the Bermuda Triangle's N9 research facility, where the Blue Noah was transformed into a space-faring vessel. This operation formed humanity's strategic countermeasure against the invading Gotham aliens.

During the Blue Noah's combat engagements and voyage, Kei Domon operates as an integral crew member. Her responsibilities encompass contributing to the ship's defense against the Gotham forces, who campaign to terraform Earth for colonization following the destruction of their own home world. The narrative incorporates interpersonal dynamics among the crew, with Kei Domon navigating relational developments throughout the journey.