From the Super Mario Wiki
Maple is a playable human character in the Nintendo 64 edition of Mario Golf. Out of all the characters who have a straight-shot, she is the most powerful. She can be unlocked by earning 50 birdie badges in Tournament Mode.
Maple wears a pink-and-white golfer top, a pair of blue shorts,and a visor with the letter "N" (likely for Nintendo) labeled on it.
Not much is known about her personality, though she seems to be very excitable.