The Mary Sue

They didn't earn high tier, they stole it.

The Self-Insert who is so perfect that they're uninteresting and boring to watch since they overcome any challenges they face with ridiculous ease. Not even in the realm of fiction would the Mary Sue's perfection be considered possible. Because they're so flawless, other characters love the Mary Sue due to their usefulness, cleverness, charisma, and/or attractiveness. The Crush of the writer, the one who the Mary Sue is supposed to act as an avatar for, may develop a crush for their character, a way for the writer to feel closer to their Fictional Love. A male Mary Sue may be called a Gary Stu.

As they're a Self-Insert, in-universe characters wouldn't treat the Mary Sue differently from how they would a Sweetheart, a role who garners similar levels of affection from others. The Poison Envy, however, doesn't take too kindly to those who are apparently better than them, so they either try to tarnish the reputation of the Mary Sue (which, Pro Tip, They Fucking Cannot) or say that they don't need the Mary Sue to get their job done. But as the story can't show any flaws in the Mary Sue's character, the Poison Envy will have to eat their words later down the line by starting to show that they had a Tsundere Relationship with the Mary Sue or the Poison Envy eventually gets in trouble and needs the help of the Mary Sue to get out of it.