Reginald Kastle

Reginald Kastle, commonly known as "Shark" (or Ryoga Kamishiro in the Japanese version), is a major character in Yu-Gi-Oh! ZEXAL.

He's introduced as a skilled but initially arrogant and aloof pro-level duelist who uses a WATER-themed Deck (fitting his "Shark" nickname, earned from his aggressive, predatory dueling style). After being disqualified from a national tournament under controversial circumstances, he becomes a delinquent-like figure at school, often acting as a rival and bully early on.

Over time, he develops a close friendship and rivalry with the protagonist Yuma Tsukumo, becoming more humble, loyal, and supportive. He has a twin sister named Rio (Marin) and uses cards like "Number 17: Leviathan Dragon" at one point.

Later in the series, major revelations show his deeper identity as Nash (Nasch), the leader of the Seven Barian Emperors, tied to the Barian World and its conflicts — leading to dramatic shifts in his role.

Overall, he's portrayed as cool, confident, and determined, with strong character growth from antagonist-leaning rival to key ally (and eventual complex antagonist figure).