For many stories, the writers include OC characters into pre-written plots, such as anime, movies, video games, etc. Although, because they add in these OC characters, they don't really know what to do with them. Let me go further into this... There are different types of these OC's and how the author creates them. Like for instance, they would write about the OC having a terrible past, which will go either they try to overcome it and be happy and make others happy, or be mean and stoic, which somehow goes out of character because they feel accepted, but what happens when they go into mid plot?

Nothing, nothing ever happens in mid plot... Why? Because the author tries to base it off of just dialogue instead of the plot and if they do try to go back to plot, it's a complete mess. Here's an example:

My name is Haru Miyashi, I'm 13 years old and I lost my parents at a young age. I was put into an orphanage at age 5. No one liked me, not even my foster mother... She beats me and tells me I'm worthless, but all I can do is smile as the pain inside built itself. Even though I dislike her, I could never hate her... She took care of me even though she refused to. 

"Haru!" my foster sister called out.

"Y-Yeah?" I whimpered.

"Come down! The food is ready!"

"O-Okay..."

She was my favorite foster sibling, she was nice to me, unlike the others. She protected me, she was my light... My only guidance.

You see anything wrong with that little thing? "No one liked me, not even my foster mother..." however, you see, "She was nice to me, unlike the others." I see this a lot in books. They claim one thing, but it changes and someone that one thing that was supposed to be part of how their life is, is now having changed into that thing. What sense does that even make? It's just awful and a mess.

Another thing I tend to see in OC based books are the OVER OC characters. Now what I mean by that is that there is more than one OC in the book. These ones try to have a group of them be together who are friends and will sometimes ship their own OC character with the other. Not to mention none of them has their own personality. They're all the same, and very irritating. Example:

"Ow! Don't hit me baka!" I shouted at Kimuro.

"Then do your work! And who are you calling baka? Baka!" Kimuro said as she stuck her tongue out.

"Guys can you be quiet please? I'm trying to study."

"But Fukuiiiiiii! Kimuro is being mean to me!" I whined.

"I don't care..."

"Haha! Fukui doesn't care about you!" Maji laughed.

I pouted at Maji and looked towards Fukui. He's such a meanie! He can't be on my side for just one second! I'll show him! I'll probably pull a prank on him! Muahahaha!~ >:3 He'll never see it coming!

The author would write that, when it makes no sense to the actual plot itself. Like it could be about someone taking someone else's pencil and they'd write a whole chapter about that and then they'd try to add something inspirational near the middle or near the end which is a cluster fuck. 

Listen, if you're going to write about an OC, think about their back story and stick to it. If there is going to be a character change in their personality, brainstorm on how that personality change, not because they said "I'm a tsun tsun, fear me!" when they really aren't, they're just an angry cutesy type that is annoying. Also, don't focus on dialogue, they don't always have to speak! *Plot is key!*

Now if you want to add more than one OC character in the story, think about their role. What part are they going to play as? What is their back story? How do they affiliate with the main OC character and the plot itself? Will they have a character change? What is their personality? Will it make sense?

These are some of the things you should consider because if you don't your story ends up sloppy and hard to read. Not to mention the overpowered OC character's, but I'll talk about that another day. For now, this is just a way to brainstorm... I guess... I don't fucking know. Who the fuck reads this anyway. Lmao