This reminds me of that one straight webtoon with a similar premise, where the MC wakes up to find herself married to a stranger who doesn't have a human face ever. And then the strange things pile up of course, but honestly, "from the burnt ashes" is already way darker than that one...
I also think this being has been watching the MC for a long time before this, possibly since that accident the MC had as a child. I mean, there's no way his abusive and neglectful patents suddenly turned over a new leaf for no reason... this is, in my opinion, absolutely the doing of the ML.
Why he hadn't weaseled his way into the MC's life sooner is a mystery though...
i thought of that story too!!
Whatever this "being" is clearly had absolutely no understanding of human relationships and boundaries.... to "it," there probably is no difference at all between siblings and lovers and friends... in any case, it probably determined that the easiest way to get itself into as close a place to the MC as possible is by becoming family. Since it's true that in society, immediate family like parents and siblings have the most importance to most people, if this being wanted to be as close to the MC as possible, creating a position as his "brother" and "changing" the world around them to fit this new reality would be the best plan to accomplish his goal.
Frankly, I think the only thing this being told the truth about is anything related to his "love" for the MC, which is also not actual love but obsession, fixation, and a desire for possession.
This is an excellent psychological thriller! Absolutely not a romance! But it's interesting so far, I'm curious where it will go!