Actor Allusion - The Serpent Undead gets hit in the face with a taiyaki plate, leaving her face messed up; this was actually a Running Gag when the actress playing her was in the role of Wendinu in Ninpuu Sentai Hurricaneger (but with less harmful implements like paint brushes, sooty explosions etc.).
Fan Nickname: Fans on both sides of the Pacific refer to Kenzaki's unseen Joker form as the Navy Joker, after the primary color of the Blade suit. The S.I.C saga DID show a Joker form for him, but it was just the same color as Hajime's. Most prefer the Navy Joker.
What Could Have Been - Each of the 52 normal Undead is pretty much this. Each had the chance to be the one whose species could've ruled the Earth, the Human Undead just managed to win out. This includes lions, tigers, spiders, grasshoppers, and even some species of plants. Yeah, Orchids could've ruled the world.