have a question about creating a multicup

How to make custom characters unlockable (by challenges) in cups/multicups? Is that possible or not?

It is possible, people do it all the time. To do it. You must have a published challenge on the your track of choice, then press manage challenges, challenge editor, then press advanced options, then challenge rewards, then you can put a a character as the reward, Then choose our custom character of Choice, and Track of Choice for that 1 character. and Wait, Stuff takes time to Validate, so that's the info i can give.

now I get it, thank you