The unlock challenge for Starman in Kirby Air Riders seems easy, but there is a catch. To unlock him, you need to finish four laps on Top Ride: Crystal in first place. That sounds simple, but if you have been trying to make it happen and wondering if the challenge is bugged, you are likely making a simple mistake. For this challenge, you must complete a four-lap race. If you do four laps in pole position in a race with more than four laps, then it will not count. I can only assume this is because they want you to have to hit first place in that very first lap. Thankfully, this is easy. Set the laps to four in the menu at the top of the screen, then set the CPU riders to the lowest difficulty to give you an easy time.
https://www.shacknews.com/article/146952/how-to-unlock-starman-kirby-air-riders
How to unlock Starman in Kirby Air Riders