Persona games offer a pretty typical JRPG experience. The only thing I feel that really makes it stand out in terms of gameplay is that they're made like old games. You know the kind, the ones where you could suddenly die and there was nothing you could do about it so you saved frequently.

If you try to rush through the game it probably won't go too well for you. Like, after defeating your first real boss in Persona 4 an extra boss appears in that area and at lv 18 it was just too difficult to take down because it would use a move that would pretty much one shot your entire party.

You acquire Persona Cards at the end of battles and then you can fuse them together to make new ones. If you register them in the compendium you can get them back for a fee.

P3 has like 9 playable characters and I believe P4 has 8(?).

The time limit is really only an issue if you are neglecting the clearing of floors or trying to unlock the ability to create the main character's ultimate version of their Persona which requires you to max out all of your social links. Bosses can also be really difficult if you aren't adequately prepared.

I think the only ATLUS games I've ever played are Persona and Disgaea though. Disgaea games are pretty much anything but dark. They have their moments, but it's mostly just cheesy storytelling and some of the most magnificent scores you've ever heard