Seiuchin is a walrus-themed Chojin featuring prominent tusks, a severely overweight physique, black beady eyes, thigh-length black boots, and a long white forehead ribbon. His design implies a suit-like appearance with pale skin visible beneath torso strings. Originally from Ireland, he is assigned to defend Hokkaido, Japan, for its suitable environment. His family includes mother Suzie, younger sister Dorothy, and late father Robert, who perished in a fishing accident. Suzie taught him to swim in his youth, and his family frequently supports him during matches, demonstrating strong bonds. Academically, he ranked second in the Hercules Factory's first-year class, excelling in martial arts theory and practical techniques.

Initially gentle, kind-hearted, and easygoing, Seiuchin performs domestic chores for friends and prioritizes their needs. He shares a close brotherly bond with Mantaro Kinniku, whom he admires and calls "Aniki." This gentle nature manifests as a combat weakness, leading him to fall for MAXman's feigned injury and suffer defeat. Despite this, he displays tactical awareness, enduring MAXman's final attack to help Mantaro analyze it. He shares Mantaro's love for food, often seen carrying and eating a large fish during downtime.

Early matches reveal a dichotomy between his friendly demeanor and latent ferocity. Against MAXman, he dominates initially with brutal techniques but loses due to compassion. During the HF First Year Replacement Matches, he fights Clioneman on the stone foot of Achilles' Heel, bolstered by family and fan encouragement. He shows resilience but is defeated after absorption into Clioneman's watery body and impalement. Later, in the Fire of Inner Strength Challenge Arc, Fork the Giant captures him, using him as a hostage to force Mantaro into a match, requiring rescue.

A pivotal shift occurs in the Ultimate Chojin Tag Tournament Arc. After repeated thoughtless remarks from Mantaro invalidating his abilities, Seiuchin allies with Neptuneman. Neptuneman subjects him to brutal training, confining him in a cave with bats that eat his body for days. This breaks his gentle personality, awakening feral instincts that reduce him to a near-speechless, violent beast. He adopts bestial traits, eating the tormenting bats. Neptuneman controls him during the tournament with a lead and helmet to prevent attacks on participants. In this state, Seiuchin participates in face-ripping incidents against Ilioukhine, Barrierfreeman, and Scarface, marking a moral event horizon and cementing his villainous turn.

His redemption begins during a match against Warsman and Mammothman. Witnessing Scarface's sacrifice to protect Jade resurfaces his original personality. Neptuneman rejects this as weakness, leading Mammothman to rip off Seiuchin's face. Hospitalized for facial reattachment, he later rejoins the Justice Chojin to apologize and reconcile. Post-redemption, he competes in the Chojin Olympics preliminaries, becoming Ireland's champion by defeating Sharkman. He is disqualified when daruma dolls collapse on him, prompting him to encourage Mantaro to win on his behalf. He later supports allies like Jade in training.

In the anime-exclusive Poison Six Pack Arc, Seiuchin participates in rescue missions against Evil Chojin who kidnapped civilians. During the Ultimate Chojin Tag Tournament Arc, his fractured friendship with Mantaro is explored; Mantaro's initial refusal to partner and derogatory comments about his utility only in chores cause further estrangement. Though external forces like the Five Disasters exacerbate tensions, Seiuchin's reconciliation requires concerted efforts from Mantaro. Post-tournament, he fully reintegrates into the Justice Chojin, symbolizing restored loyalty.