Mamoru Kodate resides as a student on Tatsumiyajima Island, attending school alongside peers like Kenji Kondou, Sakura Kaname, and Karin Kuramae. When Festum attack, he evacuates with classmates to a shelter, later finding his home obscured by newly built defensive walls. His father, Tamotsu Kodate, serves in the island's defense organization, Alvis. Mamoru expresses surprise upon learning Karin pilots a Fafner, questioning his father's secrecy. He reacts with disbelief when Sakura suggests piloting, citing the extreme risk.

He participates in social activities, visiting a dojo with Kenji and Sakura where Sakura defeats them both. Tamotsu discovers Mamoru reading manga instead of studying, returning it after asking about the story, then reveals Mamoru's selection as a Fafner pilot. Mamoru later joins friends for an Alvis facility tour, culminating in a group photo at an underwater observation deck. He is one of four candidates chosen for Fafner training before an alarm interrupts their orientation.

During combat, Mamoru adopts the persona "Goubain," inspired by a manga character whose helmet he wears while piloting. This alternate personality embodies fearless confidence, contrasting his usual self and causing post-battle amnesia of combat events. He pilots the Mark Funf, a first-generation Nothung-model Fafner designed with enhanced resistance to Festum assimilation. Its cockpit sits in the crotch area with an ejection mechanism for the control block. Initially reliant on unarmed combat, the Mark Funf later employs the Luger Lance for melee and ranged attacks. Its signature Aegis Shield, mounted on the shoulder, features four panels that deploy energy shields deflecting physical and wormhole attacks; these panels also fire energy projectiles.

Mamoru demonstrates exceptional strategic skill and combat prowess as Goubain. He eventually decides to relinquish the helmet. His friendships remain central; he questions Yukie Kariya post-battle despite her dismissal and joins peers in defying orders to witness Mark Sechs' Fenrir activation. Mamoru perishes after the Mark Funf sustains heavy combat damage. His legacy endures through later pilots; Hiroto Douma inherits the repaired Mark Funf, adopts the Goubain persona as "Goubain the 2nd," and the unit is eventually appropriated by the Neo U.N. Forces as the basis for the Mark Raison.

In later series, Mamoru appears in flashbacks and commemorations. A family photo held by Tamotsu is visible during Sakura and Kenji's wedding. He manifests among deceased individuals encountered by Kenji and Sui Kaburagi. Additional flashbacks depict him with other first-generation pilots during childhood snow play witnessed by senior personnel.