Reginald Kastle, known as Ryoga Kamishiro in the manga and Japanese version and nicknamed "Shark" in all versions, is one of the main characters in Yu-Gi-Oh! ZEXAL. 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. He, along with the Seven Barian Emperors, were revived by the Numeron Code.
Shark can be classified as a typical loner, being indifferent and cool about most things, usually sporting a stoic face or scowl. He prefers to be alone and doesn't follow a crowd, as his encounters with Yuma and others are often brief before he walks off. He also has displayed little interest in school, having had stopped going to Heartland Academy after his loss to Yuma. He still skipped class regardless when he decided to come back and would idly spend his time on the school's roof, but is said to be good at studying Despite Shark's aloof and cold nature, many students at school respect his Dueling skills and he's secretly popular with female students, however Rio mentions that he is still disliked and feared by other students Shark is shown to have a soft side, but has only shown it to two people - his sister and Yuma. Regardless of this, he can be insensitive and callous to both, causing fighting and arguing between them, especially when Rio brings up his strange fear of onions.
After being disqualified from the National Duel Circuit, Shark became bitter and uncaring, the way he held himself at the beginning of the series. Cold and malicious, Shark flaunts his power by taking the Decks of the people he defeats, making them squirm in the process, such as when he defeated Bronk Stone. When he becomes possessed by the "Numbers", he becomes quite violent, destroying the scenery surrounding him throughout his first Duel against Yuma, and also defeating several Duelists brutally when he was briefly possessed by "Number 32: Shark Drake".
However, he does have some honor, as he returned Bronk's Deck upon losing to Yuma as he promised and keeping all the Decks he'd stolen from people completely intact. His first defeat to Yuma has humbled him in a way that he is no longer overly cruel to people, as shown when he returns Yuma's pendant after winning an Ante Duel with it at stake. He also seems to put more faith in bonds and friendship, shown by his willingness to form a team with Yuma during the Tag Duel with Scorch and Chills. He further entrusts Yuma with a card that would allow them to win the Duel. Shark also tries to rescue Emperor's Key from Kite Tenjo and Orbital 7 when it is stolen, clearly stating that he sees Yuma as his friend. In the Duel with Kite, Shark remembers Yuma, and that Yuma would never give up.
Shark's friendship with Yuma also helped him in handling the "Numbers" cards. When "Number 17: Leviathan Dragon" came in contact with him, it easily took hold of him. However, he was able to fight off the influence of "Shark Drake" because of his bond with Yuma, making him the first known person to be able to resist the influence of the "Numbers" without a special item or power. Despite being able to initially resist the influence, Vetrix said that it did infect him and it will amplify his darkness caused by his desire to take revenge against Quattro. However, after a Duel with Yuma, and seeing his bumbling antics, the darkness had disappeared.
However, after accepting his fate as a Barian, he adopted his cold personality again, saying that fate drew him and Yuma together to fight against one another. Unable to consider peace as an option despite his friendships with Astral and Yuma, he attacked Earth with the other Barian Emperors. He is shown to act ruthless when Dueling, as shown during his Duel with Quattro. Throwing himself fully into his Barian persona, he incited his opponent's anger, wanting to be hated and aiming to make both himself and Quattro forget about his time as "Reginald". However, it was shown he was doing this to protect the people of the Barian World. Also when Girag appeared to be in danger, Nash immediately attempted to rescue him, showing he cares for many, if not all of the Barian Emperors (with the obvious exception of Vector). He even burst into tears at witnessing Dumon's death. He also expresses sadness at Mizar's death.
In his final duel against Yuma and Astral, Nash was shown to be grief-stricken for having failed to protect the Barian Emperors and having failed to make their dream come true. He reaffirms his vow to save the Barian World with their spirits at his side and became increasingly more desperate to defeat Yuma and Astral as they continued to survive his every assault, culminating in him adopting a crazed expression in their final exchange. He was also visibly distraught when they finally managed to destroy "CXyz Barian Hope". Nonetheless, he returned back to his friendly self after losing to Yuma and being revived by the Numeron Code.