Seiuchin is a walrus-themed Chojin wrestler, initially defined by his gentle and kind-hearted nature. He forms a deep bond with Mantaro Kinniku, viewing him as a brother, and frequently prioritizes friends by performing domestic tasks like cooking and cleaning. Physically, he resembles a walrus with prominent tusks, black beady eyes, and severe overweight. He is typically depicted carrying a large fish and wearing a long white forehead ribbon. His family consists of his mother Suzie and younger sister Dorothy; his father Robert died in a childhood fishing accident. This close-knit family frequently supports him during matches.

Trained at the Hercules Factory, Seiuchin graduated as the second-ranked first-year student. His early wrestling career centered on defending Hokkaido, an environment well-suited to his walrus physiology. He participated in significant conflicts, including the battle against the d.M.p. (Demon Seed) where he fought MAXman. Despite dominating the match initially, his naivety led to defeat after he granted a feigned request for respite, resulting in severe injury. During this fight, he endured extra punishment to allow Mantaro to study MAXman's techniques. Later, in the Hercules Factory First Year Replacement matches, he lost brutally to Clioneman through water-based suffocation and impalement on Clioneman's rib blades. Seiuchin also competed in the Chojin Olympics qualifiers, becoming Ireland's champion before disqualification when daruma dolls collapsed on him.

A pivotal shift occurred during the Ultimate Chojin Tag Tournament arc. After enduring repeated dismissive remarks from Mantaro—including public invalidation of his strength and relegation to "chore-doer" status—Seiuchin abandoned the Justice Chojin. He partnered with Neptuneman, who subjected him to harsh training: confinement in a bat-infested cave for days while bats gnawed his body. This ordeal awakened latent ferocity, transforming him into a near-feral beast. Under Neptuneman's control, he wore a restraint helmet and attacked opponents viciously, participating in face-ripping incidents against Check Mate, Ilioukhine, and Barrierfreeman. His descent into brutality peaked during a tag match against Jade and Scarface; Scarface's self-sacrifice to protect Jade began fracturing Seiuchin's altered persona. Ultimately, after reverting to his peaceful nature during a bout against Warsman and Mammothman, Neptuneman deemed him weak, betraying him by tearing off his face alongside Mammothman. Hospitalized for facial reattachment, Seiuchin later rejoined the Justice Chojin, apologized for his actions, and reconciled with his friends.

As a Chojin, Seiuchin possesses standard physiological traits: immense strength, speed, and durability. He demonstrates flight capabilities in space and can alter his size to building-scale proportions.