You can practically be any character, but not the main good guys...

Normal characters

Pacifica (Gravity falls): -Pacifica-

Bill (Gravity falls): gravity_doesnt_fall

Now with fan fic characters (yep they exist)

Bipper (Cipher falls): Gracegloom101

Jed (Cipher falls): Gracegloom101

OCs

Krystel: MEH!