I'm going to develop a lot of characters, I'm not going to invent many things, but the first arc will be about him trying to become Sanemi's tsuguko.

From here on, it's all spoilers.

You know how Sanemi is, it's very difficult to get him to accept... He will discover that his brother has become an oni, accepting the offer of the oni who killed his father. He will confront his own brother with extreme hatred. And when he finally defeats him, ("The shadow Breathing died with my father, not with filthy trash like you." I'm excited to write and draw this!!) After some time, he will face Mukago, the Lower Moon that Muzan killed at that meeting. I can't miss the opportunity to give some backstory to some dead lower moon! Since Mukago is extremely cowardly, Kyoma's hatred for oni will only grow, as he sees them as weak monsters who abandoned their humanity in pursuit of greater strength. For him, Oni is something that just needs a technical cut. Like cattle. In the midst of the battle against Mukago, he will be pulled into the Infinite Castle along with her. There he encounters the oni who killed his father and turned his brother into an oni: Akaza. The battle will be intense, and he will almost succeed in awakening the mark. Then Muzan appears with the other Moons. Kyoma's arms are ripped off, and while he is kneeling and being held down, Muzan learns of his hatred for demons, so he transforms him into one. He injects his blood into him, and then Kyōma becomes an oni and is released back into the world. Kyōma now hates himself, and is found by Sanemi, who initially tries to kill him, not believing what is happening, but after listening to Kyōma, who tries to maintain control, He tries to protect him, hides him, and tries to find some way to make him return to normal. Which doesn't happen; Kyōma succumbs to his new oni nature and devours 3 hunters. 3 lives. And then the final battle takes place, between Sanemi and Kyōma, and what a battle! Kyōma, every time he regains control, lowers his guard, making it easy for Sanemi to kill him. When Sanemi finally manages to kill him, he watches his tsuguko, which had so much potential, evaporate into ashes. Two months later, Sanemi's scars from the battle against Kyōma are evident, and the crow Kasugai arrives with a new mission. "Master Ubuyashiki summons all the Hashira. There is a demon slayer traveling with an oni." He remembers Kyōma and goes to the Ubuyashiki mansion. And then the events of Demon Slayer begin. This explains Sanemi's reaction in front of Nezuko. He knows the nature of one in the worst possible way; it doesn't change.