"You can't do that; you're going to unrealistically make the character evil if you hate someone or too perfect if you like them." Calm down; we're not writing Sonichu.
Tell me one person who has never taken inspiration from life or another source in their story. I can wait, I'll be here forever, but I can wait. Basing a character on a person isn't a bad thing. It's only bad when their purpose is to hate someone, and they have no real reason to be in the story. But you should never assume that's the intention of the writer, especially if you don't know them and have never read their work. I've been told all my ocs were projections, objectification, and hate for one person, and I won't be the last.
Realism:
One part of taking from a person is the realism that comes from them. The character inherits traits, good or bad, and that makes them feel real. You can write actions that happened in real life, and it makes the character believable. What's better? For a principal to run crying when a 12-year-old yells at him? Or for him to drag her into the office and call her parents? In real life, the second option would happen.
You can see it in Onision's work as well. When he writes a person based on a person, they are believable. But when he starts diverging from it, they start being written as unrealistically evil or too perfect for this world.
Mary Sue:
"Writing a perfect Mary Sue leaves little room for the imagination," as Kappa Kaiju says. And why does it happen? Because Mary Sue strips away the human parts of a person and leaves a husk behind. Only to be filled with how perfect she is and why she can never be wrong. They are based on a person, but the traits they're given aren't important. You can take away laziness or depression, but you can't replace it with "everyone loves them" or "they're perfect in every way." Those don't balance.
Changing Traits:
No one is perfect for the role of a fictional character. Maybe they fit a theme or a role, but they need to be changed. The school bully who thinks she is above everyone and beats up people who disagree with her isn't going to fit perfectly in any role other than the school bully. As a hero, she could learn her lesson. Or as the villain, she could be... look at Vlohran Igniseri, and you see where I'm going.
SUB, Basing from Other Fictional Characters:
The process is similar; you take a few traits from the original and add more that fit your story. You're quicker to get in trouble since you're taking from someone's creation, but if you change enough, you won't be docked points. Come on, how many characters are based on Harry Potter? You can't say "none at all."
Conclusion:
Writing takes effort and assuming a character is a "hate oc" when you know nothing of them is insulting. Writers spend days to years on their characters, and longer on their stories. So what if they base a character on a person? They're not doing anything wrong if the character is different and has a purpose in the story. Maybe it's just me, but I think it's fine to make as many characters as you want if you can work with them. No one can stop you from taking inspiration from your old mentor or the person who bullied you. If you're doing it to help your story, then what's the problem?

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Writing Tips and Reviews
RandomWelcome to Writing Tips, where I place my subjective opinion with sometimes objective facts. Whether you're writing a fan fiction with ocs or your story with copyrighted characters, tips and tricks are always helpful. Added reviews!!! I'm here to h...