From the Super Mario Wiki
The Shadow Swingstress is a character in Mario Tennis: Power Tour who follows the player's progress in the story mode of the game. She is an unranked junior player who was initially thrilled to see another unranked junior in her class, and initially assumes she can catch up with Clay and Ace. However, after she sees their talent level, she begins to ask them for advice. Her real name is unknown, but she resembles the appearance of Nina. The title "Shadow Swingstress" comes from a nickname she got from other junior players. Her doubles partner is another unnamed shadow swinger, only known as the "Shadow Swing King".
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. She seems focused, but without direction. It is implied that Clay and Ace serve as surrogate coaches once they qualify for Varsity, and give the Shadow Swingstress and her partner motivation to try to become better players.