I suspect someone hacked my account...

Because new custom characters keep appearing at the top right of my roster even though I never used them or even heard about their existence

I really hope this is a website bug and not a hacking...

You can lock them out by going to forgot password and changing it (make sure you have a email address)

I think it may be a bug, there was one incident where Senko-19 from Mudky changed into a Sonic for a lot of people, that could be the case here.

It wasn’t Senko-19 specifically, it simply replaced the top custom character on the character select menu

It's not that at all in my case, the unknown to me custom character appears in my history on top of the ones I used and can be used normally when I click on them

I remember playing this at school when I was still going, and I'd play this with friends, which is why there may be a lot of accounrs connected to this one since we played under the same IP

Maybe someone got into your IP and made an account and started messing with things?

I have no IRL friends and never logged in from a public computer