I try not to make my OC's seem Mary-Sueish. I have seen OC's in the Harry Potter fandom that screams "Mary-Sue". They always have the same backstory: Twin sister of Harry's, abused by their guardian, parents killed by Voldemort, gets sorted into Gryffindor and ends up dating Draco Malfoy as well as being the "Chosen One". The backstories look like it had just been pulled out of thin air. One of my OC's, Harold, has an interesting backstory in that his dad walked out on him, his mum and the then-unborn little sister of his. But he does not make a big deal out of it. But more on him later, I better start with the appearances of the Sues. They either wear clothes that barely cover them, have their hair in near-impossible styles, have impossible eye colour, are skinny but have big breasts. Again, I will use the same OC I used in the backstories example. Appearance: Again, Harold's appearance isn't much to write about. He has messy dark brown hair, large green eyes, his build is that he is stocky and broad-chested. He has several scars on him and on his right shoulder he has a birthmark. As for his clothing he wears for most of the time, his school uniform. Only on weekends and during the holidays does he wear non uniform clothing and usually it is a pair of trousers, a shirt and a jumper. The only other outfit he wears is Quidditch robes, and in his sixth year, fancy robes due in part of the Triwizard Tournament that was taking place then. But the Sues always has the same conditions as each other. Now, again, I'll be using Harold for this bit. Health Conditions: The Sues health conditions is never mentioned, and in the odd case it is mentioned there is no logic behind it. Now, in Harold's case, he does not have the best health, but his condition is explained. In his case, when he was in his first year he was poisoned by a poison, which in Canon, does not exist. There was fears he may not survive the stuff, but the determination of the then Matron saved him, but in the process he paid a price for it. The price was that he was left with an irregular heartbeat. Despite this he is determined to live a normal life as possible. Finally Personalities: The Sues all have personalities that makes them boring. I give my OC's a mixture of personalities, and for the fourth and final time I will give an example of one of my OC's personality. In this case, Harold's personality. He is Chivalrous, especially towards girls, intelligent, humble especially when it comes to things such as Defensive spells and brave. But he do have flaws, he is reckless, vengeful when his friends are harmed such as in one drabble I wrote when his friend was attacked by Slytherins and short tempered. But I had spent a lot of time  on this OC and it clearly shows with my comment. I would rather read a fan fiction with a well thought out character than reading a fan fiction with a Cliched Mary Sue.