For the first time ever I got past the S grade in the gauntlet!

As I check it out, I see an unknown new (=for me) icon. A kind of skull: NO CHARACTERS REFRESHED

CHARACTERS USED IN THIS GRADE CAN NOT BE USED IN THE NEXT GRADE???????????

I struggled to get so far so how do I have to pass 10 lvls while I have to save my best toons for later? Serious.