I'm not sure why I've decided to start with hair but I have.
Okay so hair is a tough one I think, because everyone is entitled to express themselves through their appearance.
Within reason.
As I said in section 1.2, having blue hair in the Heian period, Japan isn't really possible unless you were writing about some supernatural demon. But I was saying that for a human character. Even so, I'm not sure about blue since blue isn't a naturally occurring colour in plants.
Even something like hair style or colour should firstly match your setting. Most fan-fictions I see are set in modern times so this isn't much of an issue.
Anyway, this section is hard. I feel that the most Mary-Sue factors are perhaps your character has a random streak in their hair, like Ana from 'Frozen', which is caused by a curse. Or your character has completely wacko hair that doesn't fit their personality, age or other factor. For instance, my teachers would never allow me to walk around with pink hair.
That's inspired me to point out that the most Mary-Sue of them all is for this coloured hair being natural and nobody questioning it. Unless they're a different race to human, please stop designing characters like this.
Finally can I just add, stop with the blue dip dye. Okay, the odd character I can deal with but if all your OC's have dip dyed hair, just stop. There are other colours.
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Anti Mary-Sue Character Guide
Non-FictionHere is just my list of tips of how to avoid creating/writing a Mary-Sue character. Take them as you wish. Greatest Achievements: #1 in non-fiction!!!