Before I get into the topic, I want to put a disclaimer that this applies mostly to female Y/n as it is the majority in fanfics. And I am basing these tips from what I've read so far.

So, let's get to it!

1. Badass Y/n

Writing a character who is badass is not about making the character:

physically strong but is an immature sassy brat, has a prick attitude, says bad words when they want to, has a bad temper and yells a lot and gives no crap about who bullies them, and being disrespectful to other characters.

This is not badass, it's being an a**hole, or a whiny big baby who thinks being wild makes them be in control.

Being badass doesn't have to involve Y/n swearing and acting tough, or just coming up with empty sassy remarks at their bullies or oppressor. Personally, when I see these types of Y/n, I'm thinking that authors just do this to give their character personality and so that readers can relate to them.

Well authors, this is not how you do it. Keep doing this and you'll only end up creating another bratty Y/n.

Badass can be Y/n who uses her strength to fight against the oppressors and enemies because she wants to protect someone or something. (Though she can get in a fight to defend herself or because people won't leave her alone. But remember not to make her that cruel of a person if she'll deal with them)

Badass can be Y/n coming up with smart comebacks not just swear words. If she's sassy, don't pair it with her being immature and rude. Give her some manners.

Badass can be Y/n ignoring the people who keep taunting her because she doesn't want to waste her time listening to their crap and she knows that there's something worth spending time on rather than staying to entertain a fight.

(If she does get in a fight, depending on your story, I guess this is where you can also show her abilities in physical combat or her powers if she has any. But don't let her pulverize her opponents just for the sake of showing she's badass, just make her show her opponents a display of her capabilities to warn them not to mess with her further, whenever it works)

2. Y/n with a dark/sad past

We love to give our character some background for them to have more life in the story. But there's just a lot of things that I read where the "dark past" falls flat when it finally gets revealed.

Before anything else, let me just say this:

There are moments where, if you reveal the dark/sad past of Y/n, it's effective because it adds emotion to the scene or moment in the story and gives the character dept. But it falls flat when you suddenly just reveal Y/n's story out of the blue.

I read a lot of fanfics where the past of the characters are revealed too early that it doesn't really gain the sympathy of the reader. Instead, it only ends up like a person asking for attention because she has drama in life.

Look for the timing of the reveal. Once the audience finally has some sort of emotional connection to the character that makes them care about their backstory, then that's where you can make a big chunk of reveal of their past. But don't get too excited to reveal everything everything. Just things that are relevant at that moment in your story.

Okay! So, dark past....

The past is a contribution to Y/n's very being and helps shape the person she is.

So if you're going to give her a dark or sad past, don't overdo it. Don't be so cruel to Y/n that you're going to make her life a complete living hell and then expect her love interest to fix her. Don't do that.

Also, people with a dark past can still have positive attitude and behavior, they're not all emo, depressive and cold.

Whatever the pain you're planning for Y/n in her background story, you have to have a better understanding on how it may affect her in her present life or what it did to change her. What it did to make her think the way she does, or what it did to make her scared of things.

Understand the effects of whatever dark/sad past that you're going to give Y/n so you can design and then build her character as your story goes.

(This can also apply to characters in novels and originals. I just wanted to include this for fanfic characters)

3. Y/n's powers/ abilities

Just one thing for this: Don't overpower Y/n and then make her weak against enemies that she can actually handle with her powers.

Please.

And if you planned powers for her that doesn't fit in the anime universe you chose for your fanfic, think twice.

For example, in a Hunter X Hunter fanfic I read years ago, Y/n suddenly has the power of the four elements like the Avatar. If you're going to do something like this, you would have to rattle your brain to make sense of this because you are using the universe of the anime. If it were in the ATLA or TLOK universe, or crossover, or an original story, it's less problematic.

By the way, that four element powers was revealed twenty chapters later in the book (if I remember correctly). If you do this, it only shows that you didn't plan the story properly and you're randomly throwing in these things out of the blue just to move the story forward and that's a big mistake. You'll sabotage your own work.

Establish the powers of Y/n in the first place so that you won't confuse your readers by throwing random powers out of the blue. And it should one way or another go with the magic system of the anime you're using.

4. Y/n's personality

I've read a lot of Y/n who is almost the same as another Y/n from a different story which makes it boring because it's like you're reading the same person in a different anime fanfic.

"Isn't that a good thing? At least it really feels like you're that same person but you just traveled to another anime. It's like the point of X Readers!"

Wrong.

Yes, it is a reader insert story, so there are thousands of readers out there with different personalities. And not everybody is that one bratty, boring, annoying, and selfish Y/n.

Don't create Y/n to be like most of the characters that already existed. It's okay to make Y/n a bit similar to them but not almost like them.

Vary Y/n from others with a realistic personality. It's fiction, yes, but to make a character interesting and relatable, you have to make them act as close to real as possible. At least there will be many readers who may relate to the Y/n character.

Reminder!

You are giving the chance for readers to be someone else for a while, so give them a decent and interesting character for them to imagine themselves as.

That's all for now! Hope this helped :)