Seiya Kanzaki, known as Seiji in the English adaptation, is a member of the Four Kings—a subgroup within Team Shark Tooth (or Team ST). The Four Kings serve under Team Shark Tooth leader Gabu Samejima, functioning as supporting antagonists in X-City. They exhibit unskilled riding abilities and depend on underhanded tactics like cheating and race interference to disrupt opponents such as protagonist Sho Yamato and his allies. Gabu directs their actions to maintain control over X-City and obstruct Sho's gold emblem collection.