Gaidel is the father of a girl named Leona. He was a member of the Hakkesshu, and an avatar of water who ran away from his destiny as he did not wish to destroy humanity. Like he did with the rest of the Hakkesshu, one day, Leopold Goenitz visited the village where Gaidel, his wife, and Leona were. Goenitz asked for help to ressurect Orochi, but Gaidel refused. Goenitz left, but returned later that night, and activated the Riot of the Blood within Leona, who killed her father, her mother and everyone else in the village.
In the Ikari Team's ending in The King of Fighters '97, Gaidel was seen talking to Leona and told her not to follow Orochi and his destiny, but to follow her own fate. Leona cries and thanks him for his advice.
In The King of Fighters XIV, it's revealed that Gaidel was one of the souls inside Verse that returned to life after the monster was defeated, although his whereabouts are unknown.