Kouyou Kasugai resides on Tatsumiyajima and becomes a Fafner pilot. His surname blends "spring" and "well/mine shaft," while his given name signifies "grade A/first rank" and "ocean." Gentle and introspective initially, he adopts a stray dog named Chocolat and carries unrequited feelings for Shouko Hazama, who is bedridden before perishing in battle. His adoptive parents display minimal concern, celebrating his Fafner pilot selection despite his aversion to combat.

In the original series, Kouyou joins the first generation of pilots. During an early island scouting mission, Festum assimilation damages his central nervous system, inducing a coma. He later awakens as a Slave-Type Festum, preserving fragments of human consciousness. This state enables him to interface with Festum networks and perceive phenomena beyond human senses.

During *Exodus*, Kouyou returns to Tatsumiyajima as a human-Festum hybrid. He intervenes critically in battles: destroying an Azazel-type Festum underwater to rescue Sui Kaburagi and shielding Mitsugu Jinnai’s aircraft from pursuers. Visiting the Hazama family grave, he glimpses visions of Canon and Shouko, solidifying his resolve to protect their home. He retrieves Canon’s hat and delivers her final coordinates to Orihime Minashiro.

In *The Beyond*, Kouyou pilots the Mark Vier Kai, Abaddon, during the Fifth Operation Azure. He employs his SDP to divert Festum attention toward himself, declaring they should target him to "learn about death." He later confronts the Specter unit, identifying its pilot as Maris Excelsior and deducing their time-buying objective. After the Sixth Operation Azure fails and Tatsumiyajima is reclaimed, Chocolat passes away peacefully. One month later, during the Tanabata Festival, Kouyou departs the island with Kazuki Makabe to explore the world.