I just got her from the Special Pipe in Mario Kart Tour:
The Special Pipe doesn't have any spotlights, so I could have gotten LM King Boo instead (it was a 50-50 chance). I'm glad that I got the character I wanted, and I can't wait to unlock Halloween Peach soon!
I think he was even hit by his own teammate one time, since he's so tall and he's usually near the net.