I'm somewhat new to Feralheart, so I haven't had any cringe-worthy characters inside the game, but I still have a lot of cringe-worthy characters from roleplaying in general.
One of the most notable was Juniperstar from a Warriors LARP with my friends. I was still fairly new to roleplaying, so I wasn't familiar with the term 'Mary Sue'. Juniperstar was a black cat with bright green eyes, who was the leader AND the medicine cat, delivered a plot-convenient life-changing prophecy at a time of need, fulfilled said prophecy, saved all the Clans, died, and then was revived as a ginger tabby 'with one black paw, to remember who she formerly was'. Juniperstar went down in fake, noncanonical Clan history.
I also had Shadewhisker, who I actually reused as a character multiple times. Shadewhisker now isn't too bad (although I suppose I don't really do Warriors RPs anymore), but the first Shadewhisker... oh boy...
Shadewhisker was grey and black with purple (yes, PURPLE) eyes. She had wings (in Warrior Cats) and a forked tail, and ALL DA 'PAWS MADED FUN OF HER. She was the typical 'pity me, I'm so sad and bullied and my abilities are a curse'-type character. Not to mention she had a sister (Lilydusk) who was 'loved more than her' and 'did so much better' and 'won at everything'.
I could go on for hours about my countless terrible characters I made when I was little, but I won't. G'bye.