Seihai-kun (Grail-kun) is a parody character in Carnival Phantasm. He is a mascot-styled version of the Great Grail. He was originally part of the TMitter April Fools' Day site where he was listed as (せいはいくん, ?).

His segments in Carnival Phantasm follow characters seeking help with their problems, and Grail-kun giving them a tool like the "Friendmaker" and the "Hero Creation Kit" in a Doraemon-like manner. Each tool is just a simple kitchen knife, and he implies that they use to solve their problems in violent manners like "show him who is boss" or "you'll become a hero after killing a million people."