Sounds like this https://www.mangago.me/read-manga/tatari/
I was gonna say https://www.mangago.me/read-manga/to_you_the_immortal/ but it’s a boy and a dog
Yep, that's it! Thank you!!!(๑•ㅂ•)و✧
It's not this one but you just made me remember this masterpiece existed, I actually have the physical copy but it's been a while since I last picked it up to read it, thanks anyways! (≧∀≦)
I haven’t read it but seen the anime, just done first chap now and am a blubbering mess
So, I remember that MC was a cat and he was being taken care of by a boy that was super poor but then the boy dies and he becomes the boy. Yep, that's all! Also, he was probably a youkai the whole time but I'm not sure... ╮(￣▽￣)╭