From the Super Mario Wiki
The headmaster in Mario Tennis
The Headmaster is the head of the Royal Tennis Academy and chairman of the Island Open in Mario Tennis for the Game Boy Color and Mario Tennis: Power Tour for the Game Boy Advance.
In Mario Tennis, Alex or Nina first arrives at the academy and gets bumped by the Headmaster who then apologizes, welcomes the new student and then departs. After Alex/Nina has completed the Varsity matches, their roommate/partner informs them to head to the Headmaster's office. Inside the office, the Headmaster congratulates the player for their achievement amd wishes them good luck at the Island Open, the Headmaster escort Alex/Nina, Emily, Kevin, and Mark to the Island Open. When they arrive, the Headmaster gives them an auspicious remark, and heads back to the Academy. During the time in the Island Open, the player can visit the Headmaster inside his office. He later appear at the ceremony given out the trophy to winners for their success in the tournament. After the ceremony, the player meets the Headmaster inside his office who notifies them that Mario has invited the player to a match at his World.
The headmaster introduce the students to the Island Open in Mario Tennis: Power Tour
In Mario Tennis: Power Tour, he is first seen talking to Alex after the mysterious masked players came. He also comes to take Clay, Ace, Elroy, and Tori to the Island Open in doubles mode. When they first arrive at the Island Open, the Headmaster burns from the heat. His assistant, Samantha, pours a buck of water on him to cool him down. After playing in the "Mario World", the player can visit the Headmaster in his office in the main building. The Headmaster wears a green suit with a red tie. He has black hair and a small mustache.