The first girl we got introduced to is his sister, Claire Kagenou. She has a part of the soul of Aurora sealed in her arm; Aurora is the name of the violet-haired woman who later became the demon lord Diabolos through some means (against her will). MC calls Aurora Violet.
Then there are the 7 original girls.
- The Shadow Garden leader, Alpha,
- the author, Beta,
- the clumsy leader of the chocolate/bank/etc corp, Gamma,
- the no brain all brawn wolf(?)girl, Delta,
- the musician, Epsilon,
- the spy-master, Zeta
- the sleepy researcher who wants to do human experiments, Eta.
These have rescued other girls. The amount of girls was too much (>600) for Greek letters, so most have a number and only the strong or special ones have a Greek letter as a name.
It has been revealed that Zeta disagrees with Alpha about the course of Shadow Garden; she instead wants to do something to make our MC immortal or smth. She has formed her own faction to push this goal. Members of this faction include some of her subordinates that are also in Shadow Garden, like Mu (Greek letter), but also some that are not in Shadow Garden, like Nina. Nina is the gyaru and is Claire's best friend and secret bodyguard.
Then there are noble girls. At some point, Cid confessed to princess Alexia Midgar, who accepted a fake relationship and calls him Pooch. She has twin tails.
Her older sister is princess Iris Midgar, the student council president (or maybe she gruadated by now) who leads the Crimson Order. She thinks Shadow Garden and Diabolos Cult are different names for the same thing, and hence considers both enemies.
Bordering the Midgar kingdom is the Oriana Kingdom. Their princess Rose Oriana (with drill-hair) joined Shadow Garden as nr 666 (after the Shadow piano scene) before becoming the queen (and cutting her drill-hair). This is the kingdom Alpha is busy developing.
More recently, we learned of the noble Christina Hope. She is the one who so creepily smiles at the Jack the Ripper kills, and who forced Cid to sleep in her room. She has no history before this arc, as far as I know; just another classmate. (EDIT: We first met her when MC took the appearance of her cousin Suzuki Hope, who died as his collar exploded. She was one of the people who watched as he transformed into Shadow and defeated Fenrir, the wolf-like bad guy.)
EDIT: During this Fenrir+Hope arc, we did meet another noble girl who was making classmates kill each other for fun and was taking their mana so she would stay alive. Her name is Eliza Dakuaikan, and she is the one we wanted to be judged which led to the current arc.
I think this covers most important characters.
