Keith Water serves as a Fafner pilot under Dustin Morgan in the Human Army. During preparations for the Festum siege of the Neo UN Hawaii Base, he casually jokes about greeting the enemy before launch, revealing an informal demeanor under pressure. He pilots an Olympia Angels unit, the ground combat variant of the Powers Model Fafner, surviving both the initial Festum assault and the subsequent ROE Alpha nuclear strike.

Reassigned to Argos Company, Keith pilots a distinct black and grey Gabriel-type Fafner, an aerial combat Dominions Model variant. He participates in covert operations, including an ambush targeting Nothung Model Fafners and Srinagar Base survivors. During this mission, he provides overwatch support with an Improved Garm 44 equipped with a high-powered scope, delivering precise fire that severely damages Akira Nishio's Mark Zehn while covering Dustin Morgan's Raphael unit.

Keith demonstrates pragmatic loyalty to commanders and mission objectives. Defending Neo UN Headquarters, he engages Maya Toomi's Mark Sieben, but his Gabriel unit is disabled after Mark Sieben severs its wings and legs using wired projectiles. His final deployment occurs during the Human Army's assault on the Alvis-03 Island facility, codenamed Atlantis. Attempting to destroy Mark Sieben with support from Billy Morgan's Ariel unit, Keith activates the Fenrir Program—a remote self-destruct system—on his Gabriel. When Mark Sieben temporarily disables Fenrir using wires, Keith exits the cockpit to physically destroy the disabling wires. Though knocked away by Mark Sieben, the Fenrir Program reactivates, vaporizing both Keith and his Fafner. His tactical approach emphasizes ranged combat and strict adherence to orders, particularly evident in his overwatch role and compliance with directives to eliminate assimilated targets.