From the Super Mario Wiki
Kid (Kiddo) is a young golfer from Mario Golf for the Game Boy Color and Mario Golf: Advance Tour. He also appeared as an unlockable character in Mobile Golf. He had apparently started playing golf ever since he was three and had already become a golfing prodigy at age ten.
In Mario Golf, he was one of four characters that the player could choose from at the beginning of the game and train however the player like. He could then be uploaded to the N64 version for more training.
In Mario Golf: Advance Tour, it is established that Kid had become the Grand Champion in the last game. However, he was injured badly due to excessive training caused by strain of the muscles and joints to the point where he could not even grasp a golf club. It was said he could no longer golf and lost his title to the former champion, Gene. After a five year recovery, Kid became an avid golf trainer to Neil, Ella, Buzz and Helen, until the Club Manager influenced him to pick up the club again when he signed them up in Marion. He ascended the ranks quickly and took back the championship.
Kid lead the title over the Links tourney. He also ended up forming a doubles team with Gene to advance up the doubles (more likely for the benefit of Gene gaining another title).
Kid never seems to lose his cool, and always tries to be fair to his friends and his foes. He encourages his students to keep going strong, no matter what. He is a very respected golfer and is considered to be just as good, maybe even better, than Mario.
In Mario Golf, Kid is a young boy with kinky brown hair and blue eyes. He wears a white-collared black polo shirt, blue shorts, and a white glove on his left hand. His socks are white and his shoes are copper orange with white stripes. Over his hair, he wears a red baseball cap flipped backwards.
In Mario Golf: Advance Tour, Kid has grown older, taller, and his features have matured. He wears an indigo-collared maroon polo shirt, a black belt, and white pants. His golfing shoes are black and indigo, and he still wears a white golfing glove. He now wears an indigo baseball cap forward, with a red square in the center.
 Mario Golf
 Mario Golf: Advance Tour