Using Freeform Universal rules to desgin an OC
People who feel like they're having a hard time grasping the essence of their OCs (like I often do) I can't recommend enough to check out Freeform Universal RPG Classic Rules by Nathan Russel. Seriously, even if you're not into TTRPG, check out the character creation rules.
In FU you don't roll for stats, spend points on skills or buy equipment and calculate encumbrance, you just make a series of decisions about your character. It's basically a guide to how to turn a simple concept into a draft of an OC with strengths, weaknesses, relationships and at least one personal plot ready to run in a very short time. It can be an infinitely useful tool for writers.
You can grab FU for free here: https://www.drivethrurpg.com/product/89534/FU-The-Freeform-Universal-RPG-Classic-rules The whole document is 23 pages with the character creation rules being solid 5 pages. You don't need any of the other stuff if you don't wanna actually play.
To show what I'm talking about, I'm going to make an example and show how quickly you can build a viable fictional character from a simple concept with these rules.
For simplicity, my OC will be called Adrien (he/him). He's a High School Demon Hunter. While I obviously have some vague associations of what kind of guy he might be (I imagine him kinda like the deuteragonist of a shonen anime, someone like Ishida from Bleach) I'm gonna work out all the details on the fly in this post.
Descriptors are the core of all FU characters, taking the role of abilities, skills and the like from other RPGs. What makes FU distinct and very useful for writers that you can pick any descriptor and the rules guide you to highlight the most iconic traits of your character, making them more rememberable than a bunch of stats.
There are four Descriptors: Body, Mind, Edge and Flaw.
I'm not sure about what kind of physiche I imagine for Adrien, so I consult with the example Descriptor list in the rules. I know I don't want him to be all about muscle, but he needs to be somewhat fit to keep up with the demons supernatural tricks, so I pick Vigour. He's not ripped or anything, but he's quick and has good stamina. Thinking about how could I incorporate this into his characterization other than showing him dodging magic blasts from demons, I decide that he jogs every day.
It's a much easier choice, as I know that I want him to be a good student and someone who fights the supernatural relying on his knowledge and wits, so I pick Studious. He does well in school and in his freetime he keeps learning about demons and the ways to defeat them.
Adrien knows a lot of incantations to ward off, weaken and banish demons so I pick Arcane Knowledge. I could just call it Magic, but I want to emphasize that he's not your artillery wizard, blasting fireballs back and forth, he just knows a lot of spells against specific enemies and situations.
I want Adrien to wear glasses that can reflect the light, hiding his eyes whenever he's in "serious mode" so I pick Nearsighted as his Achilles' Heel. It can also make for good complications, since if he looses his glasses he'd be in a serious disadvantage in battle and other situations. (As you'll see shortly, I could just make Cool Glasses a piece of Gear for him, but this way it's more integral to his character)
Now you pick two items that are iconic to your OC. Think about things like the Doctor's Old TARDIS and Sonic Screwdriver, InkSans' Giant Brush and Magical Vials, you get the idea.
For Adrien I pick Ancient Talisman, a mystical trinket that will help to amplify the power of his spells againsgt demons, and Clean Shirt, a pick of clothing that will help to establish him further as a trusthworthy and honorable student (even if he happens to be found in places where he's supposedly has nothing to do as such).
This is basically the motivation of your character, a short- or longterm goal they want to achieve. In other words, their personal plot. What is great about FU's approach is that it divides your motivation into three questions:
What do you want? Your end-goal.
What's stopping you? The source of the conflict. After all, if you could just reach out and take your goal it wouldn't be mmuch of a plot.
What will you do? Now that you know what your character want and why can't they get it right away, time to figure out what they wanna do about it. Once you know that, you basically know where to start to write their story.
Adrien is here to kick demon ass and get good grades, but let's make his goals a bit more exact than that.
What do you want? The Cult of the Blood Moon (I'm improvising here okay?) keeps summoning demons to this world, constantly endangering people's lifes, but their real goal is still unkown. I have to stop them, no matter what it takes!
What's stopping you? The members of the Cult keep their identities in secret and they seem to have almost infinite resources and powerful connections. They think they're untouchable and they're not far from being right about that.
What will you do? If I keep thwarting their operations, eventually they have to come out from the shadows. Then, I take them down once and for all.
There! You just got a plot!
Characters don't exist in vacuums, their connection to others what really shows who they are. For playing an actual RPG adventure, this is where you figure out how your character is connected to the other players' characters. For making a fandom OC you can use this part to connect your OC to the canon characters. And for an original work, this is the time to establish some supporting cast, other characters you can detail and develop further as the story goes.
FU suggest creating two Relationships for start, so let's do just that.
Harold is my mentor who taught me everything about fighting demons. His knowledge and experience is invaluable, I just wish he trusted a bit more my abilities and decisions.
Liana is my little sister. I hate to lie to her about where I'm going every day after school, but knowing about the demons and what they do to people would freak her out so much. I just want to keep her safe.
And that's pretty much it. I forgot to check the clock when I started to write this, but it certainly didn't take longer than a hour with taking some short breaks. And as I said, I'm someone who is tipically struggles with outlining my OCs. Seriously, if you find yourself blocked when you try to flesh out an idea for a character, check out FU. It won't do the heavy lifting of writing for you, obviously, but it gives you a fram to work with and a great foundation to build upon. And it's like, totally free. Even if you find that it doesn't work for you as well, it costs literally nothing.