From the Super Mario Wiki
Maple is a human character in the popular video game Mario Golf. She only appeared in the Nintendo 64 edition of Mario Golf. Out of all the characters who have a straight-shot, she is the most powerful. Maple wears a golf-type top colored pink and white, a pair of blue shorts and a visor with the letter "N" labeled on it. This likely represents Nintendo.
She can be unlocked by earning 50 birdie badges in Tournament Mode.
Maple seems to be very fun. When she is selected, she gets very excited. She shows it by doing a dance. Not much is known about her for Mario Golf was her only appearance.