Cliché ass backstories.
You know, the typical:
"I was left in the doorsteps of (insert surname of probable future love interest) household when I was only a baby. The parents took me happily. But as soon as I grew up, my powers developed. They then hated me! Tortured me! They didn't accept me for who I am! I was waaayyyy stronger than my clan! (Lol I randomly added that their with no roots to it whatsoever) I became emo, I dyed my hair purple, I was depressed. I'm shu foken saddo. But only their child was kind enough to accept me. We grew up and I fell in love with him/her. And we lived happily ever after"
That didn't continue on as a backstory lol idc.
So technically, as soon as that plot is getting really overused; it gets boring as shit.
So create originality! Go do it! Have a happy childhood with a sad twist or a sad childhood with a happy effect to your character! It may improve your character as soon as their story progresses on!
The key point to this is: "Originality"
Most Mary Sues/ people who make Mary Sue ocs tend to go with the cliché overused plot points to their fanfiction or just a regular story they created with that oc.
Oh and this,
If you're gonna have that kind of backstory it'll most likely end up with a cliche present personality for your character. But Mary Sues? Nah.
Depressing backstory, parents die, grows up to be 2009 emo scene kid who only listens to Evanescence. Likes "Bring me to life" unironically (lol wat). Goes all happeh happeh with main love interest. They then fought side by side in the samurai fight of world war II in Afghanistan. And soon fell for each other. The sakura petals falls serenely down to the ground as Nagisa-jackie confesses her love for Sousuke-cum "s-s-s-s-sen-p-p-p-p-pai n-n-o-o d-a-i-s-u-k-i-i-i-iiiii" Sousuke-kum declines. "I'm sorry my childhood fren / possible love interest in the canon story / nobody ships except the author. I love Nekamoli-jackie."
Sousuke-cum said. Leaving the poor loli in tears as she ran away with her annoying high pitched voice.
Now that's a cliché storyline.
Lol what did I just do ??
Lolz idek ??
(I say lol ironically)