Seiya Nishikage is the goalkeeper for Outei Tsukinomiya, later serving as a reserve goalkeeper and temporary defender for Inazuma Japan. He possesses gray eyes, a tall stature, a muscular build, and light brown hair typically swept upwards with one loose strand framing his face.

Nishikage exhibits a quiet, unassuming personality with minimal emotional expression but demonstrates unwavering loyalty and dedication to Nosaka Yuuma, whom he views as a heroic and authoritative figure. He consistently accompanies Nosaka, analyzes soccer strategies with him, and conforms to all of Nosaka’s decisions without objection, even consuming excessive food at Nosaka’s request despite personal discomfort. This loyalty stems from his admiration for Nosaka’s lifelong dedication to a singular goal. Nishikage reacts with intense anger and sadness upon learning from Mikado Anna about Nosaka’s brain tumor, feeling betrayed by Nosaka withholding this information. He remains deeply protective of Nosaka’s well-being, confronting him about the illness and insisting Nosaka should not play in his condition. During matches, Nishikage displays frustration when unable to stop shots, fearing he is unworthy of playing alongside Nosaka. Despite his reserved nature, he develops amicable relationships within Inazuma Japan.

In "Inazuma Eleven: Ares no Tenbin," Nishikage supports Nosaka in scouting matches involving Inakuni Raimon and Seishou Gakuen. As Outei Tsukinomiya’s goalkeeper, he effortlessly stops Haizaki Ryouhei’s Overhead Penguin using basic saves and his signature technique Ouke no Tate, contributing to Seishou Gakuen’s forfeit. After learning of Nosaka’s tumor from Mikado Anna and overhearing Nosaka’s medical discussions, Nishikage confronts him, though Nosaka dismisses his concerns. During the Football Frontier finals against Inakuni Raimon, Nishikage initially struggles—failing to react to a feint leading to a goal and being overpowered by Kozoumaru Sasuke’s Hidaruma Bakunetsudan. He adapts, later stopping the same shot with Casting Arm and collaborating with teammates to block Counterdrive using Ouke no Tate. He is ultimately defeated by Goujin Tetsunosuke’s Fire Lemonade, resulting in Outei Tsukinomiya’s loss.

In "Inazuma Eleven: Orion no Kokuin," Nishikage is selected for Inazuma Japan and assists Nosaka during his hospital discharge. Initially benched behind Endou Mamoru, he starts as goalkeeper against Uzbekistan’s Eternal Dancers when Endou is arrested. He concedes two goals to Dost Gale’s Requiem Dust due to defensive disorganization and expresses frustration over his perceived failure. Nosaka’s unexpected return at halftime revitalizes Nishikage; he apologizes for the conceded goals and vows to protect the goal. Using his new technique Artemis Ring, he successfully stops Requiem Dust, earning Nosaka’s praise. Analysis suggests Artemis Ring’s effectiveness relies on Nosaka’s presence on the field, indicating a limited range. Nishikage later participates in training simulations to avoid fouls and supports Nosaka’s tactical plans, including fabricating injuries for Fubuki Shirou and Kazemaru Ichirouta to expose the Orion Foundation. He shows subtle jealousy over Nosaka’s collaboration with Ichihoshi Mitsuru but rejoices when Ichihoshi and Asuto resolve their conflicts. In the match against China’s Soccer Acrobatic Troupe, he collaborates with Endou and Saginuma Osamu to execute The Asura, stopping Tenkuu Hayabusadan.