What Makes A Mary Sue or Gary Stu

I've seen way too many people make Mary/Gary Sues. I want to help put a stop to this, so here we go.
 * Has a TRAGIC past (parents dying, being abused, losing loved ones, being cheated on, being bullied, etc)
 * Having ALL the powers (fire/water/darkness/immortal/etc)
 * Being part EVERYTHING (part wolf/cat/goddess/bunny/dog/hamster/angel/demon/etc)
 * Being absolutely perfect/no flaws at all
 * OR having false flaws
 * They are loved/hated by literally everyone
 * Absolutely beautiful/handsome
 * Extremely humble despite being loved by literally everyone
 * Extraordinary talent
 * Cheated on multiple times over and over again


 * Assumed nobody then learns that they're the "chosen one" or being perfectly sane then killing everyone

It's fine for your OC to have powers, or be pretty, or to be loved by a lot of people, or to have a sad past. But try to make it at least a bit believable.