FatFluffyPenguin

As I guessed, it's Alice in Wonderland/Through the Looking Glass. Quick explanation if you don't know: We first knew 3.2's Dark Side was based on Alice in Wonderland/Through the Looking Glass (which I'll just call AiW) from the achievement you get from entering it (Teenager in Wonderland) and also from the White Rabbit (that has pink contrails when it moves) within it, which I guessed was Denia basically as soon as it appeared. With her reveal though, plus my analysis on the katashiro and her involvement with the Dark Side (TL;DR is that she can most likely manipulate it as she controls negativity/sin/the energy of the Dark Side), it's basically confirmed that she's based on Lewis Caroll's story. In Wonderland, Caroll takes a lot of inspirations on playing cards but I don't see them a lot here. One of the things you'll see in this reveal is chessboards. It's in the background as well as on the cake surrounded by katashiro. This is because in AiW, the story is symbolically framed as a game of chess, something that was confirmed by Caroll himself. The whole story of Looking Glass structures itself on the characters being chess pieces, and Alice being a pawn. However, Denia is either the White Rabbit, the 'lure' into Wonderland and the Dark Side/Void (also a servant of the Queen of Hearts, the villain), or what I think is more likely, the Mad Hatter as he appears in both books (Wonderland and Looking Glass), Alice's accomplice and someone who provides a very famous riddle: "Why is a raven like a writing desk?" The riddle is nonsensical and has no answer, and is the lesson to Alice that not all questions have answers. I think this links quite well with Denia's quote here, "Do not watch, do not listen, do not commit to memory... Some things are best left unknown."