Ok though my Japanese is not that good but basically [spoiler alert]
Our MC (Hikaru) actually has existed for a long time, and actually a kudan. And there's a rule that only one kudan should appear in one era, but as Hikari doesn't fulfill his role as a kudan, there's another kudan born in same era and then it's turned into a disaster/curse. So this thing is not the seven character's problem anymore, it's a catastrophe against humanity.
People who gathered in the power or sight of this Kudan fall one after another and die.
And so Hikaru tried to stop the another Kudan to save the world.
And then the ending: The world has been rewritten. The accident in first chapter didn't happen, all of the characters are alive and don't remember anything from the previous world. AND there's no Hikaru in this world and no ones remember about him. Then we got a little time skip. Chizuru is a working woman now. She got so busy and stressed out with her work, so she and her senior visited some jazz bar. The bartender recommends cocktail to the stressed Chizuru and then introduce himself as Hikaru a.k.a our MC. THE END. Sorry for the messy writing and grammar, it's 2 a. m here. Didn't bother to proofread lol.
You guys can read the last chapter here: https://comic.estar.jp/comics/11252576/viewer?work_id=24781767

Just finished reading the RAW. Hmmmm the ending is kinda meh.