they're kinda more features of individual arcs rather than part of the basic premise of the story so you don't need to worry

This story is a fusion fantasy that takes place in the kingdom of Silla of ancient Korea. There is some esoteric terminology for the supernatural beings, the heavy Buddhist influences, as well as other societal positions and minutae that doesn't translate conveniently well to anything in English. Although Korean readers have cited the novel as being well researched, there is not much extended in-novel exposition for this for better or worse, choosing to explain certain concepts as they come up but not overly so. The lack of a glossary to explain many of these translation choices and their nuances does make it difficult to follow what's going along when it comes to the intricacies of the ghost stories and their resolutions, which might make some of these mysteries feel confusing or unsatisfying to an English reader who lacks background knowledge.

My personal opinion is that the emotional payoff of most arcs was worth the stumbling in the assembly of the building blocks, but I am aware I am a bigger picture over the sum of the parts type of person. While I liked the ghost stories, they were mostly quite straightforward, generally structured more to set up a specific emotional response rather than encourage the reader to solve anything. If you were looking for something very innovative, there was probably only one ghost tied to the main character's past with a gimmick I thought was rather unique, but the rather unclear translation makes it difficult to fully parse all the moving parts of that arc. If you believe this might be a dealbreaker to you, this is my warning to you. As long as one or both of the main characters interests you from early on, I encourage people to give it a try nonetheless even if the start seems a little bit hard to follow along. There is a turning point in the story around the fourth or fifth labelled arc that may change your perception of the main characters and the course of the narrative.

There are undeniably two main characters to this story of equal importance, but because the entirety of the main story is told from the point of view of Seol Yeong, I will consider him the protagonist for convenience. Seol Yeong is a hwarang who practices sorcery and can listen to the voices and regrets of the dead. He has bad history with the rest of the hwarang outside of his faction, and although he has been diligently serving as a hwarang for seven years, he is inconveniently framed by a bad omen on top of his existing poor reputation and willingly gives up his position with his faction temporarily to act as an independent hwarang to clear his name. Although he is doted on by and equally loves his brothers, he is quite acerbic, willful, and antisocial to basically anyone else. The novel itself is written in a way that feels quite dry and curt, which I believe matches the tone I think Seol Yeong would take in his head if he was narrating it from third person. Even though he may have honourable and earnest intentions, Seol Yeong starts off with many enemies and few allies on his journey to resolve the growing number of incidents across the kingdom.

The deuteragonist, Jaha, is the former leader of the hwarang, who mysteriously disappeared eight years ago after defeating the previous Great Calamity. While all of the hwarang except Seol Yeong (who joined too late to recognise him) respect him deeply for what he has done to save the kingdom, he has a rather difficult personality, and teases others with the playful arrogance of someone who is used to always getting his way. When he reappears out of the blue only to start travelling alongside Seol Yeong, he's initially quite actively antagonistic and patronising, although he occasionally steps in to help Seol Yeong out at crucial moments, seemingly on a whim.

Seol Yeong is ostracised from other people on a more obvious level, because of his blunt attitude, strange upbringing, and past conflicts with his fellow hwarang. Jaha on the other hand, has been put on a pedestal as a person born to be a hero for much of his life. He is able to make objectively tragic decisions and sacrifices with the understanding that no one else but him is in the position to do so, and initially bears the heavy burden of both the admiration and hate with lofty grace and cold indifference.

Although on the surface the two main characters could not be more different, I liked that they continuously ended up relying on and saving each other to the point that they trusted and understood each other better than anyone else could. Seol Yeong, because he is uniquely qualified to empathise with the dead, is the only person who understands the weight of Jaha’s sacrifice as well as the grief and resentment he could not show to the rest. In turn, he keeps Jaha's secret and stands by Jaha's side, because he sincerely believes that through their combined efforts, they will be able to overcome any hurdle. It is Seol Yeong's words that allow the jaded and isolated Jaha to continue hanging onto things like hopes and a future, things that he had already given up on after his first failure. I personally thought it was very touching that they grew increasingly indebted to each other as both literal and metaphorical lifelines to each other.

Perhaps the thing that hits hardest about Requiem is once it sinks in who exactly the titular requiem in question was for...

For people who may want to ship the protagonists, you of course can (there is quite a large overlap between overseas fans of Requiem and danmei titles), though some people might be unhappy with how the story ends with a bittersweet and open-ended reincarnation hook. On the other hand, if you're already familiar with common danmei tropes, it might make certain people even more into it...