The Spider and Her Demons is really cool and unique, with characters who are sort of darkly fucked up but in an intriguing way that makes you eager to dig out all their secrets throughout the story.
Dior, the possible-friend-possible-danger, is strange and kind of off-putting and definitely dangerous, but she's also charming and charismatic and her her attempts to kindle a friendship with Zhi feel like they stem from genuine loneliness. The disconnect keeps you on your toes with regards to whether or not you can trust her, and even if you do decide to trust her the story won't let you forget that she's capable of some pretty fucked up shit. Being me, I naturally appreciate a morally complicated character who's neither demonized not excused by the narrative, and as such Dior and her relationship with Zhi were an excellent focus.
Being me, I also really liked the themes of friendship over explicit romance (though Zhi and Dior's dynamic can certainly be interpreted as romantic). Many of the books featuring aromantic or asexual characters that I've read have only a brief namedrop of the labels, often in the form of an off-hand joke (an exasperated 'I'm too asexual for this' is a common one), while never actually exploring the identities or how they impact the characters. The Spider and Her Demons, however, never actually use either term, but it does heavily discuss friendship and romance and how the two are pitted against each other, and the general feelings the characters have about not wanting to be in a romantic and/or sexual relationship. It comes off a lot more genuine than books which treat throwing labels around like checking off a list. Here, it is an integral part of the narrative and Zhi's character journey as someone who can't/won't ever have romantic and/or sexual relationship.
The main flaw of the book is the plot and lore. The main focus is the tension and building relationship between Zhi and Dior, which is done excellently, but there is also the mystery surrounding Dior's nature, about what the rules for the supernatural even are in this world, as well as a more action-driven demon vs. demon hunters sort of sub plot. While these were interesting ideas, I felt they all needed more work done. The demon and magic lore especially felt very half-baked.
This is an issue I often see in smaller, more character focused stories that also decide to add in some bigger, outside plot. The plot ends up feeling disconnected and underwhelming because the character beats are so much stronger and better developed, and the attempt at a big epic climax that largely builds on something completely different will ring hollow unless that aspect (in this case, the lore and Dior mystery) are given equal depth.