The other anon pretty much explained everything. You didn't ask about mechanics or gacha so I'll omit those.
If you're interested in the protagonists I can tell you some things without spoiling too much. From a hentai standpoint the 12 girls fill diferent fetishes (loli, femdom, corruption, gyaru, cosplay, masturbation).
From a personality standpoint they are also pretty varied in the usual cliches (genki, diligent girl, tsundere, kudere, otaku, tomboy, gyaru again). They have their twists however (like in madoka their magical girl transformations are manifestations of their shadow self). Each girl has some deep trauma or obsession that allows her to transform. The story and bond scenes explain them in detail if you're curious.
Their scenes and dresses are usually related to these. As an example Seira fills the diligent prim and proper student but her sex scenes usually involve degradation and corruption. Lilly looks like the slut gyaru type but her prefered scenes (when she's not getting raped) are of tender lovemaking. Some are more straightforward. Marianne is a degen otaku so her scenes tend to imitate the eroges she plays all day.There are exceptions and every girl gets her vanilla time but if you look close enough you can see the patterns. Funnily enough Iroha has one of the less varied scene roster in my opinion. She fills the vanilla type role. Some scenes get her forced but not to the extremes other girls endure (yet). But like Hanabi it's only a matter of time she gets some.
And yeah there's lot of rape. Parallel worlds mean there can be failure states where the girls lose and get broken completely without compromising the story as a whole. Also it makes the events and collabs easier to explain. You can check the demon tower scenes and some events for those bad end type scenes. There's a torrent with pretty much every scene >>394253