Shadow Swing King
From the Super Mario Wiki
The Shadow Swing King is a character in Mario Tennis: Power Tour who follows the player's progress in the story mode of the game. He is an unranked junior player at the Royal Tennis Academy who was pleased to see another unranked junior in his class, referring to Clay and Ace, but quickly discovers their talent and begins to ask them for advice. His real name is unknown. He wears an orange shirt, and has brown hair. His doubles partner is the Shadow Swingstress, another unranked and unnamed player.
Both players are unable to practice on the courts due to being unranked, so they are consistently seen swinging their rackets at the air, as their unique form of practice.
Other people in the junior class have made comments about their level of play. Sylvia and Sasha comment when playing against them that they are easy opponents to defeat. It is implied that Clay and Ace serve as surrogate coaches once they qualify for Varsity, and give him and his partner motivation to try to become better players.