Why Mary Sue

ocs that insert themselves into already existing lore and mess things up suck, eg. _____'s sibling/partner, _____'s classmate, or a really op oc that messes up the power system. Like you cant be Naruto's sister because if that were canon, he wouldn't have gone through all that bullying, loneliness and trauma and it just ruins the plot. You can't be a dsmp's partner because 1) they are actual people and its weird, 2) their characters are probably too traumatised or worried about war to care about romantic relationships and like their personality would just mess up if they had an actual trustworthy person around them (these points relate to most characters but not all) You can't be Bakugo's girlfriend because 1) You would think Bakugo would hate shy people and probably tell them death threat, i mean in the first episode he literally told Izuku to go off himself. 2) Would he really care about romance? Like the only thing thats probably going on in that head of his is being the best, hero stuff, school and getting stronger. Mans has priorities and getting a girlfriend isnt one of them. (Note: I'm no longer into anime so im not caught up with any plot)