So how many mafia numbers are there? I think Kira and Misa are the only mafia but there 18 players here so 2 is too little.
Somebody has answered it earlier but for an 18-player game like this, the ratio should be ideally:
2-3 indies, 3-4 mafia, 11-12 townies. Remember this is a role madness game and scums need firepower in order to dominate the town.
As for the characters who might be scums, if following the above format
2 indies: Higuchi (you're right) and/or could be Kira himself. We are talking about a mass killer (genocide psycho) who will try to kill anyone who will be a hindrance to his so-called Justice.
Mafia - Ryuk, Rem, Misa-Misa!, Kira may also be in this line-up tho.
We are already playng the game but it's surprising that you asked this and yet you did nothing to push forward. That an indicator. -1 point for you.