*Reginald Kastle, known as Ryouga Kamishiro in the manga and Japanese version and nicknamed "Shark" in all versions, is one of the main characters and the final antagonist in Yu-Gi-Oh! ZEXAL other than the unseen threat to Astral World.

He is one of Yuma Tsukumo's close friends and rivals. He was originally an arrogant and callous person, but has grown quite humble. He is later revealed to be Nash, the leader of the Seven Barian Emperors. Determined to not let tragedies befall his people again, he betrayed Yuma and his friends to safeguard the Barian World.

He is the only Barian Emperor not to be absorbed by Don Thousand, as well as the only Barian whose original memories were not altered. He later vanished after losing to Yuma in a Duel that had the Human, Astral, and Barian Worlds at stake, but was revived along with the rest of the Barian Emperors by the Numeron Code.*