Walter Barghest serves as a military pilot embroiled in conflict zones during the series’ events. When ordered to execute ROE Alpha by bombing an area containing active troops and civilians, he openly defies the command, brandishing a firearm at his superior in protest. This act triggers his immediate discharge and arrest, though the aerial strike proceeds.

Later assuming command as a second lieutenant and captain of the Fourth Garrison, he coordinates with Tatsumiyajima’s expeditionary force in Srinagar, prioritizing caution under ROE Alpha to prevent friendly fire during Festum incursions. After observing Kazuki Makabe rescue assimilated pilots, he reacts with raw emotion, tearfully acknowledging the unprecedented survival of casualties.

His past resurfaces when confronted by Akira Nishio over his role in Tatsumiyajima’s bombing three years prior. Admitting he acted under orders, Walter vows to atone by dedicating himself to a renewed purpose. A tentative reconciliation follows, culminating in Akira extending an invitation to visit the island—an offer Walter accepts, longing to see Tatsumiyajima directly.

During subsequent battles, he aids in defending survivor convoys against Festum swarms and Human Army assaults. In a pivotal clash, he triggers the Fenrir program, fusing his Fafner to an enemy unit and detonating both alongside Neo UN submarines. His final words mourn his failure to fulfill the promise of witnessing Tatsumiyajima.

Other details highlight his role in exposing a spy within the survivor camp, leading the traitor to ingest poison upon capture. He also fosters gradual trust with former foes, sharing camaraderie through gestures like offering Akira coffee and inquiring about his amulet.