He is depicted as a narcissist as a child and his appearance closely resembles his mother.
He has yet to make a physical appearance in the main series and only appears through flashbacks.
Tobaku Datenroku Kaiji: One Poker-hen
He is briefly shown in flashbacks towards the end of One Poker when Kazuya began thinking back at his childhood to a specific event that changed his life.
While the family was on a yacht, a wave suddenly hit knocking Kazuya and Kazuki both from the boat. Due to a string of bad luck, Sophie only noticed and rescued Kazuki not realizing Kazuya was also knocked off due to his position on the boat when the wave hit.
Kazuya also believes their father favors Kazuki over him due to his prospects.