Rittenhouse absolutely is a public figure.
Public figures also include individuals who have achieved pervasive fame or notoriety. A person can even become an “involuntary public figure” as the result of publicity, even though that person did not want or invite the public attention.
Read Gertz v. Welch, Time v. Hill and Dameron v. Washington Magazine.
I looked up all 3.
It looks like Kyle Rittenhouse unfortunately meets the "public figure" standard.
Bubba_Leroy's post pretty well summed up the difference between Kyle Rittenhouse and Nick Sandman and the Covington High School kids.