Heads up that I may end up mixing details from multiple different stories because I have a terrible memory and I also read a lot of similar novels BL where MC shou transmigrates into a novel/video game/whatever, maybe bc he was pissed at how the novel ended MC transmigrates into baby/toddler who is picked up by the ML gong in the slums and is consequently raised by him The MC's mother gives the ML an item containing the elf queen before the mum dies They meet and befriend a young girl in the slums who's dad own a brothel The MC is often mistaken for a girl, sometimes for the ML's sister or daughter Turns out MC's mum was a run away from a famous clan and had famous friends Slum gets obliterated by some ancient evil with them being the only survivors End up going to some kind of magic school MC at some point gets kidnapped and trained by his uncle (i think) when he's older Cross dressing ML seduces a noble to take one of his magical items and then sets fire to mansion to cover his tracks
...from your memory I am reading the book to reading list just for the sake of crossdressing ML moment