Mafia 21.

You don't seem very cheerful, Mold)
Many people write like that, almost in every mafia game.

Yeah, my sister had a meltdown here, that's why I'm not cheerful -_-

and I still don't understand who Kosha was.

Let's figure it out.

I was digging for clues, and it turned out to be a calendar. Here's what we have:
No first page => The first page is January => no one was born (of those who indicated a date) in January, didn't register on the site then, and isn't a member of a clan that was founded in January => let's look further => it turns out that *Worms Armageddon* was released in January! => There are grounds to suspect Registra.

But, it's quite possible that the clue points to someone else, and we need to check other versions.