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One of the five True Magicians, the "Magician of the Second Magic" and also one of the founders of the Clock Tower, holding the title of Wizard Marshal in the Mage's Association. Zelretch is mentioned and briefly appears in many of Type-Moon works, notably Tsukihime and Fate series, but his origins and details as to how he reached the Second Magic are unknown.

He's the creator of the Mystic Codes Magical Ruby, Magical Sapphire, Jeweled Sword Zelretch, and Keitai-san.
In Fate/Grand Order he appears as the illustration cover for the Kaleidoscope craft essence. Given its popularity, he's also known as "Kaleidoscope" or "the Kaleidoscope guy".