I know about most of the characters in this game and, if it helps, I can tell you about them.
PLUM: Peach's daughter, I think. Has the weakest drive, 208 yds. You start off with her.
CHARLIE: Appearantly having a bad hair day. His drive's better, at 210 yds., but it curves a little. You start off with him.
PEACH: A pretty good character for beginners.
She has a drive of 212 yds., and, best of all, you start off with her. Not bad, eh?
BABY MARIO: A very strong...crier. Takes after Mario a lot. He has a slightly curved drive of 215 yds. He's a starter, too.
LUIGI: An OK golfer, he is still hidden in Mario's shadow. He's 40 yds softer than Mario (230 yds.) He's in Get Character Mode.
YOSHI: A straight-forward dinosaur, Yoshi is one of the best golfers in the game. He must be earned in Get Character Mode. He drives 230 yds.
SONNY: I have never seen him in any other game before. He drives 240 and is a Get Character Mode player. The whole encheladda.
WARIO: He might just've found his sport here.
His ball is electric and goes 250 yds, so I believe he must be cheating. He's a Get Character.
HARRY: Not much to say. He drives 260 and is a Get Character. He's pretty easy to earn. But his goatee's cool. I wish I had one.
MARIO: Yet again, Mario's got his own game, and is playing to his fullest. With a 270 yd drive, he's a force to be reckoned with. A Get Character as well.
MAPLE: Better than Peach and Yoshi combined, she's a 1 in a million player. To get her, simply earn 50 Birdie Badges from the tournament mode. She drives 245. Did I mention she's good?
DK: Oddly, this muscle-packed primate plays golf with one hand (a.k.a. he's strong). Get 30 Ring Shots completed to earn him. He's a long driver at 275. He puts Mario to shame!
BOWSER: His motto is "give up or get out". His short game isn't very good, but he makes up for it with a 280yd drive. I still haven't beaten him!
METAL MARIO: I have no idea whatsoever about Metal Mario except you need every Birdie Badge
to get him. And he's made of metal, so his drives must be long.
There's also 4 extra characters you can earn in Mario Golf for Game Boy and transferred to the N64. They remain unknown. Well, ciao!