Jean-Claude Magnum is a Hollywood actor who has starred in several Ninja Movies. He was a suitor and opponent of Mai Valentine. His name is a reference to actor Jean-Claude Van Damme.
Sometime in the past, Mai defeated him in a Duel. Impressed, he proposed to her, but she walked away in disgust, telling him to ask her again when he could beat her in Duel Monsters.
Magnum entered Battle City and won five Locator Cards. Needing six to qualify for the finals, he challenged Mai to a Duel for a final Locator Card and the right to marry her. When he lost, he used Hollywood stunts and special effects in an attempt to kidnap her. He was stopped by Joey Wheeler. This confrontation revealed he was a coward at heart. When Mai was in danger, he panicked and asked for his stunt double to help him. After Joey rescued her, Mai berated Magnum and ordered him to leave, and he reluctantly did.
Voice and Mannierisms
He speaks very calm. In the japanese version he uses the personal pronoun "watashi", also, he uses the american expression "D'Oh" (from "The Simpsons") when frustrated. Which may be a representation of his american origin or just a coincidence.