Kouyou Kasugai, a Fafner pilot and student on Tatsumiyajima Island, balances his gentle nature with the burdens of duty. His empathy shines through his care for a stray dog, Chocolat, and his quiet devotion to Shouko Hazama, a bedridden classmate whose unrequited affection deepens his solitude. Emotionally detached adoptive parents celebrate his selection as the Mark Vier’s third pilot, amplifying his isolation through their focus on honor over his safety.

Initially reluctant in combat, he avoids direct confrontation and recoils at battle footage until Shouko’s death during a mission ignites a fierce resolve to protect others, transforming guilt into purpose. Assimilated by Festum during an expedition, his physical recovery masks a consciousness fused with their networks, awakening as a Festum Core. Gifted with immortality and enhanced powers yet estranged from humanity, he continues piloting the upgraded Mark Vier Kai Abaddon, honoring his vow to shield Tatsumiyajima.

In battles, his tactical acumen shines—rescuing allies like Sui Kaburagi from assimilation and confronting Azazel-class threats. His Super Dimensional Power, “Poison,” enables his Wormblade to corrode Festum, crippling their regeneration. Beyond combat, he perceives metaphysical traces, detecting captive children within Festum systems and bridging communication with fallen comrades such as Mimika Mikagami.

Ultimately transcending human limits, he operates as an autonomous Festum Core while remaining Tatsumiyajima’s steadfast protector. Partnering with Kazuki Makabe, he thwarts Legato’s advance and joins the Fifth Operation Azure. His journey concludes alongside Kazuki, venturing beyond the island—a testament to his metamorphosis from a hesitant student to an eternal guardian existing between worlds.