Is only the Storyteller allowed to play NPCs?

[Asriel Dreemurr]

I should like to play some Smash Up characters where I, Jak, and Jay are located.

[The Rogue]

Nope you can play an NPC of you need to, unless it is an site NPC, then you need to ask

[Kakashi Hatake]

Connor is correct. The Storyteller account is typically used for reactions to player activity or exposition for Omniverse events. You're free to RP random NPCs as you please, so long as they are secondaries. If they're Primes, they'll need to be an original character or from your character's source material and approved beforehand for record-keeping. Secondaries don't require approval, but if you want them to be long-term companions we appreciate a heads-up. If you want to use a site NPC (Dumbledoor, Thrall etc.), drop a request in the NPC Roleplay Requests thread beforehand for approval.