Reginald Kastle is the final antagonist of Yu-Gi-Oh ZEXAL other than the unseen threat to Astral world. His nickname is "Shark" due to having a WATER Attribute Deck. He is a close friend of Yuma Tsukumo, but was a bully before they became friends with each other. Later on he is revealed to be Nash (Nasch in the Japanese version), leader of the Seven Barian Emperors.
He was voiced by Toshiki Masuda in the Japanese version, and Gary Mack in the English version.
In his past life, he was known as Nash the king of his people who ruled the United Lands of the Poseidon Ocean alongside his sister, Marin and was loved by his people, one fateful day a mad prince known as Vector attacked his kingdom with his men and few Gorgonic Guardians which were to stone by Nash & his armies mirror shields.
Vector then performed a blood sacrifice to Summon the sea God Abyss to gain the upper hand, Marin rode up to Abyss and was grabbed by the God which the knight and his horse Pegasus failed to do and she offered her soul to summon another God to Purify Abyss, after Marin's funeral Nash ordered his men to mount another attack on Vector & his army in which they suceeded, meanwhile they stumbled upon a village that Vector had already destroyed and they met a girl that resembled Nash's sister who introduced herself as Iris and he cared for her.
later on he went to confront Vector alone and avenge his sister Marin, Vector challenged Nash to a Shadow Game in which Nash won and he witnesses his men & Iris had died, then Nash followed Vector back to his palace and defeated him again and witnessed the people that Vector killed drag him down to the afterlife.
In the past they were enemies who fought in an all out war, and when they were reincarnated in Barian World Vector often clashed with Nash's leadership, then at a certain time he decided to betray him & Marin and attempted to get rid of them both succeeding with the latter and near with the former. In Nash's duel with Yuma, Nash honors Vector keeping his monster's overlay unit on Barian hope and fighting as allies in the series finale.
During his tenure as Emperor of the United Lands of the Poseidon Ocean, he had befriended a knight from another kingdom. This knight, Dumon, had become close with Nash, swearing to defend and fight alongside him and his enemies. Their friendship even continued on to their next lives, when the two men had been reincarnated as Barians where Dumon would continue to serve Nash once again. Even after Nash had supposedly disappeared from the Barian world, Dumon (now leader of the other Barians) held on to the hope their leader would return one day. Once Vector had mentioned the possibility of their lost leader being on Earth, Dumon became absolutely determined to find and bring his leader back. Once it became apparent that Nash was indeed the human Shark, Dumon was able to convince him it was his destiny to join the Barians, and finally Nash had joined up with him once again. They fought as former barian emperors in the series finale once they were revived.
At first, Shark had no knowledge of Astral, but Astral has respect for Shark's Dueling skills and the impact he has on Yuma. If Shark becomes possessed by a "Number" card, Astral and Yuma usually come into conflict with Shark and try to save him from the card's dark influence. Sometime between Shark and Yuma becoming friends and the World Duel Carnival, Shark's learned of Astral's existence and knows how important he is to Yuma. Knowing Shark is trustworthy and strong-willed, Astral has no problems letting him keep "Number 32: Shark Drake" card in his possession. After the Duel with Vector, Shark can now see and hear Astral, but they still have little contact with each other.
Once Astral's theories about Shark and Rio being Nash and Marin were confirmed true, he readily accepted that Shark was his enemy when he saw him with the Barians. Despite beings enemies now, he showed concern for Astral when he attempts to sacrifice himself.
- During the Duel with Abyss, he is shown memories of his and Rio's past.
- In the subbed version he claims that when he became a Barian his former self died.
- Just like Yuma's Shining Draw, he has the ability to perform Chaos Draw.