All the persona users after the initial 4 are included in future updates for free, but characters like Kazuki and Aiko/Akari are paid DLC. You can also get all the NPC characters as navigators for two dollars each.

I'd personally recommend getting Aiko ASAP as he's top tier and consequently used online a lot, and figuring out how to play him will work wonders. Saya's also top tier but she's pretty easy to understand so buying her isn't necessary if you don't want to play her.