This is a side-quest introduced in Volume 2. In order to complete this quest in its entirety, you must:
* Complete first in all five courses.
* Complete 50 races.
* Collect over 3,000 Chim Chim Spheres over the course of racing.
You can check your progress by talking to Kafu in Dol Dona, just across from the warp point near your new @Home. While the second and third might be the easiest, keep in mind two things for the first requirement: a way to clear them easily has been posted for Lone Racer, and you get parts going to your Book of 1000 for clearing them, too. There is an order to difficulty for the courses, too, passed down from the Playstation 2 era: 4, 3, 5, 1, 2, from easiest to hardest.