Seiji is a member of the Four Kings (Shiten'nō), a group within the Shark Tooth MTB team, specifically identified as "Sea roach Seiji" (フナムシのセイジ, Funamushi no Seiji). The Four Kings are unskilled riders who specialize in underhanded tricks and interference during races. They act as a collective unit, exhibiting a fearful and subservient demeanor around their leader, Gabu Samejima, feeling confident only in his presence but otherwise intimidated by him.

Following Team Shark Tooth's arrival in the X-Zone, Seiji and the other Four Kings supported Gabu's dictatorship in X-City. When Gabu abandoned them, Sho Yamato rescued the group from individuals controlling dark emblems. This led Seiji and the other Four Kings to defect from Team Shark Tooth and ally with Sho to battle Team X.

Seiji's background prior to joining Team Shark Tooth, his individual actions outside the group dynamic, and specific development beyond the main series remain undocumented. His role throughout their documented appearances remains consistent as part of the Four Kings collective.