I rarely review and critique novels, so bear with me here.
I mainly looked at the prologue for this critique and the first two chapters. Another thing I want to get out of the way first it that, I can tell that you want to instill mystery about the characters involved by not giving distinct descriptions or names at first and I'm going to get into the issue I have about that later. And lastly, I'm going to assume that English is not your first language, because it doesn't feel like it is, but correct me if I'm wrong.
I like the premise of the novel, it feels like it has a lot of potential for action and mystery and drama. But what it has in potential, it lacks in execution.
Overall, the prologue was very stilted and awkward to read. There were multiple typos and the syntax (sentence structure) isn't well done. This prologue lacked suspense, action, and drama because at no point are we indicated to whom we, as readers, are supposed to side with, the soldier or the oni. The soldier also doesn't struggle against the oni. We never feel like that the soldier might fail, making the scene boring to read. And it's all over very quickly too.
The descriptions of the characters, and this continues in the other chapters, are kinda generic or nonexistent. Now I realize, some authors don't like describing their characters and that's fine, but for me personally as a reader, I prefer for some type of indication of what characters look like. The soldier is described generically as being "covered in armor and cloth." No indication of the color of the cloth, the patterns, the type of armor, what the armor is made of, if the armor is battle-torn, if the face of the soldier is obscured. Nothing.
The main oni fought at the end of the chapter, is also described generically (and there's a typo). There's nothing to set this oni, of this story, apart from other generic, bargain bin devil/demon-like characters other than the hat and coat but then again, we aren't told what kind of hat or what kind of coat. This is set in Japan so I assume the hat and coat are Japanese in origin (maybe a rice paddy hat?) but for the uninitiated, the first thing thought of might be a top hat with a wide brim. If you are going to describe things and characters, do you due diligence when doing so. You don't have to get into long descriptions, but give more to the audience so they can understand and imagine things in their heads.
This chapter featured something that really drove home the idea that the execution of this novel is lacking: you telling the reader that Kagami is an outcast rather than showing the reader. Now I recognize that you wanted to keep the reason Kagami is an outcast a secret, but that doesn't mean you can show us through his interactions with other people. I suspect that was you goal with his interaction with his father, but rather than showing us the degree of separation between Kagami and others it just came off as you showing us the awkward father-son relationship between them. Or maybe that was your goal all along.
One thing that showing us Kagami being an outcast would've done, is endear the reader to the character. There's not much going on with Kagami as of chapter one, and being isolated from other people is a common feeling many people have gone through. If you had shown how other don't interact with Kagami, that certainly would get more people invested in his story.
Kagami is also not described, at all I'm pretty sure. Again you don't to stop the story to give a character description, you can weave it though out the narrative, only mentioning details where necessary.
"Kagami's dark colored eyes twitched with annoyance after hearing the favor his father asked of him."
Speaking of, you don't really describe character action or reaction either.
The thing that stuck out to me the most was the lack of continuity and the repetition in the dialogue and narrative.
Firstly, the continuity: Kagami is laying down in the grass at the start of the chapter. You mention he looks out at the field, which is possible to do from laying down position, but then you say "[He] sat back down against the grass..." When did he get up? It's not too big of an issue, but not being able to keep track of the characters' locations and positions are going to make future scenes, specifically action scenes, that much harder to follow.
Onto the repetition: thrice you say "He looked (or 'looking') out into the field" and twice Kagami starts a sentence with, "I bet..." Vary your sentences more and don't fall into the trap of repetition. Repeating phrases and words like that comes off as amateurish and are tedious to read.
One other thing, don't be afraid of establishing shots, or in this case, paragraph. Set the scene. if it's nighttime and there's a festival in the distance, the reader should be allowed to experience that. Imagine how enthralled the reader would be if the second chapter started like this:
"High in the sky sat the crescent moon, it's cool, thin line of light glimmered on dark, grassy fields. The glow of the moon fought for attention with the fiery and bright blaze that was the festival's light. Decorations of every color hung from stalls offering food, games, and masks. The people bussed around, enjoying the night with their family and friends. They forgot about their day jobs and hardships as they listens to the high pitched, upbeat music and and danced to the rhythm. Everyone in town was basking in festival fun. Everyone but Kagami. Kagami, instead was company to himself and the crescent moon. Only he reveled in its glow, because he laid alone in the grassy fields watching the victimized sheep. He glanced beside him, observing the orange glow that burned into the darkened horizon and thought to himself 'I didn't even want to go to festival. It's not like I'm needed there, anyway.' He rolled over in the grass, facing his back to the festival, but then he heard something among the blades of grass."
Sorry, I got a little carried away. I recommend writing more establishing scenes. They help portray the mood and emotions of scenes while also giving the reader a chance to be transported into the world you're writing about. Now not every scene needs an establishing paragraph, but not having any at all can leave the reader feeling uninvested and confused. We don't even realize it's night until the third paragraph of this chapter! I assumed it was evening time, not night.
Again typos, At some you have a sentence that says, "The French were thrown back into the hill with great force, barely missing Kagami."
I had to pause for a minute because I was thinking 'the French? I thought we were in Japan? Oh it's a typo, they mean the fence'. You might want to add another round of proofreading before posting so you catch mistakes like that that pull the reader out of the story.
So I think overall, the issues that I see are the typos, awkward and tilted sentences, a lack of varied diction, a lack of establishing paragraphs, little to no descriptions, and the chapter lengths.
Typos: Try and get someone who's a native english speaker to read over your drafts, pass the drafts through proofreading site like Grammarly, or read your drafts out loud. These are all way to catch small mistakes that go a long way in making your novel feel more polished.
Awkward and tilted sentences:
This is something reading your words aloud might fix.
A lack of varied diction: There's a lot of repetition of words and phrases. If you ever find yourself repeating words, consider opening a thesaurus. Try to only use repetition sparingly and where it can make a point in the narrative.
A lack of establishing paragraphs and little to no descriptions
These are kind of the same thing. There's not enough describing going on, not of the setting or the characters. And when you do describe things, the descriptions come of as dry and generic. For example, how you describe the oni as having "blood, red skin." Trying to find different ways and words to express comparison in your descriptions. Instead of "blood, red skin" what about "skin dipped in blood" or "skin as crimson as a fire," just something different.
This is something small I'm bringing up at the end, but your chapters are very short. And their brevity feels less purposeful and more like you don't know what else to say. You're not establishing your scenes, you're not describing your characters, you're not getting into the action, you're not getting into the emotion of the scene or characters. Things come off as flat and boring, when there's much more beneath the surface. It feels like I'm reading a rough draft, a first draft, rather than a final draft.
I'm sorry if this seems like a lot or if any of this comes of as harsh, I don't mean it that way. There's plenty of potential for the story to be great, but it's going to take a lot of hard work to get there. I commend you for not only putting your story out there, but also asking for critique on it. I hope this helps you when writing future chapters and good luck!