Basically, you have a roster of 2-4 characters, and each character has their own set of possible cards (and passives, stats, equipment slots, etc.) and their cards go into a global pool which is drawn every turn. Cards have their own mana cost, and you're encouraged to use cards from different characters each turn as most cards will cost +1 per card from that character used that turn.
I can't tell if this is stupid or genius. Making classes only matter for choices you get, and everyone can do everything in the end is...interesting.
It sucks that the game is bad, though.
I was trying to find a way to keep the description succinct (and it didn't help my original writeup got wiped), so it seems I cut it a bit short:
There's an non-com fifth member of the team that you play as, and their personal skills are all related to card draws; you're commanding the others to use their skills. Every turn you can choose to toss one card in the discard and draw a fresh card, and variations of this mechanic are crucial to a lot of specific boss mechanics.
Each combat character has their own hp bar, stats, a unique passive, and each time you level, you get to add one (or two, for the first level up) of their personal cards to your draw pile, and those cards
are unique to them.
For example, one support character's passive gives you -1 card draw, but in return, you get to pick one of 2-4 cards to be added from your draw pile to your hand (so you get the same number of cards per turn, just one of them you pick), and a lot of her personal cards get a major buff if you choose them. Some effects include it playing for free, getting to pick another card, forming a barrier, etc. Another character gets a defensive boost and counters any attack with her topmost attack card in your hand when she's hit. And, one weird character that I can't quite figure out how to properly use, is literally invincible, but it's a game-over if they're the last survivor and their cards are deadweight unless you keep feeding them food... which is in fairly short supply.
There are also neutral skills that can be learned from books in-run, and you can actually transfer those between characters freely (so long as they have the requisite 5-skill minimum).
Another feature I forgot to mention-- you're also prohibited from infinitely chaining skills since there's a counter on mobs that show how long you have before they act (it's kind of like Time Eater's limit, except it doesn't end your turn). Kind of annoying, coming from StS, but I think it actually feels better this way.
(As far as it being
bad... Steam likes it; it's clocking 90% positive after 200 reviews. I just think there's enough roughness to elements that it can't get wholehearted support and is instead, like most of what I feel like commenting on, a tentative positive. Though that's because anything strongly negative I mostly just bin and don't feel inclined to say anything about, unless it's a flaming dumpster fire.)
Update (01/06/2020): So having spent some more time with this, and having finished the available stages a few times, I have to amend myself that the major problem seems to be that some of the mechanics are just godawfully phrased, and when it can take upwards of an hour and a half* to reach a boss, you're not very generous when it comes to losing due to a poor description.
And sometimes the descriptions are downright inaccurate:
"Instantly shoots the enemy twice when he deals damage on a
single enemy with his
skill from the hand twice in a row" (quote and emphasis directly from the game)... Based on that, it sounds like he's all about deck manipulation to draw/use as many of his cards in one turn. Not. At. All. He's actually a chaser; have somebody else attack, and him follow up to trigger his special.
And for fun, a more complex multi-line mess of a description (less relevant bits cut): "Dark Heal" "Deal the maximum damage of (X) equal to the enemy's lost health." "Heal for the maximum amount of (X) and increase Pain resistance if the recipient is an ally." "Target : Pain Resist Decreased ... Pain resist -12%"
You can use it on your own party on on enemies, with it healing or dealing damage. The attack does up to X damage based on percentage of enemy life missing. So it basically sucks against HP sponges. And that bit about Pain Resist increasing or decreasing... It increases it when used on your own party. Or at least, the buff says it's increased. Who knows what it's actually doing.
*I never managed to finish it in less than ~90min (I think my best was 1:26), even though there's a few guys on Steam who're like 'What? It takes you over 40min a run? Well, it's cool if you like playing slow.'