Introducing... YAMAS SUPER SIMPLE GUIDE FOR MAKING OCs THE YAMA WAY. I'll be referencing my old wiki page, Yamashita Ritsu for the duration since that's the most fleshed out OC I've made.
I always start with creating a character based on the weapon they use. For me, my OC's weapon is an extension of themselves and serves as a general theme for them to be based around. I created Fainarugado, Yamashita's Zanpakuto, way before I even created Yama. I knew I wanted a defense oriented weapon that wasn't a shield but was still deadly in the right circumstances. In it's infancy, lets' call it Fainarugado V1, it originally only had 1 ability and in short, it charged energy the more attacks it blocked, and then could release it all in a large energy attack. So with that, it's theme was "Defense/Offense" or "Counterattacking". I mocked up a little picture, gave it a cool fitting name (Fainarugado = Final Guard) got's it's ability figured out and I was set. This also helped to determine my OC's fighting style which should be an extension of their personality.
With my OC's weapon / Fighting Style done I was ready to move onto the OC to fit that weapon. First I thought about what kind of person would PICK that weapon. Like if Fainarugado was sitting in a shop, who would buy it? Considering it's theme of defense, and it's main ability involving blocking a lot and then counterattacking I knew my OC had to be stubborn. Giving up wouldn't be an option. To be effective with the weapon, they'd have to tough out difficult fights with hopes of using the one trump card they had. So that weaved it's way into his personality. Stubborn, Doesn't give up, steadfast. Check.
Next, I thought about how they would act towards other people. Someone stubborn wielding a defense oriented weapon with a deadly counterattack after a long battle. With this in mind, I made Yamashita initially a little more reserved, solitary, and hard to get things out of. He would be nice up front, kind, but keeps people at arms length. Very guarded of his emotions. So even out of battle he still fits the persona of a guard as he protects himself and his feelings while also trying to protect others from getting too involved with his personal matters. Friendly but reserved when interacting with others. Check.
To fit the theme, I also made him quite protective of others, especially those weaker than him but that's kind of a given considering his theme so far. This also loops back onto his stubbornness, given that he would protect anyone from any threat even if the threat was far stronger than he.
So with that, we now have a defensive oriented fighter, who is kind to others but keeps his distance, guarding his own emotions. He vows to protect anyone, especially the weak, no matter the odds. Never gives up on a fight.
Now I'd look at those traits, and find what flaws they would cause in his personality and actions. Given his defensive oriented nature, he is not a very skilled fighter when on the attack making him lose most battles where he's forced into an offensive position. The stubbornness of him as well as his counterattack based fighting style puts him in many high risk high reward situations and even clouds his judgement at times when facing immensely stronger adversaries. He doesn't know when to run causing him to get injured regularly and have a high chance of death. His inability to open up to anyone makes finding friends hard and leaves him short on loyal allies.
Okay so now his traits are both good and bad in certain situations.
Now we have "Guard-Type OC". We know what kind of weapon he/she uses. We know how they fight, how they act when fighting, and how they act towards others. All of this without even making a name or even figuring out what they look like.
This step is optional, and not super necessary. All that I did here is flesh out a very basic back story that explains why the OC acts the way it does.
For Yamashita I needed a reason for why he acts the way he does. A lot of it is up to you, I just pieced a general idea starting with "He lost someone he to protected". Given this simple 6 word phrase, I now have a sort of 'Motive' to his personality and actions. So I made up a little bit about him having a sister and how she was picked on and he was always there to protect her blah blah then he left to be a Shinigami and couldn't be around for her. She ends up dying and now Yamashita blames himself for not being there. Now we have reasons why he's reserved (so other people close to him don't get hurt) why he's stubborn (cause he can't live with losing anyone else he was supposed to protect). This whole thing also still fits with his "Defensive" theme Zanpakuto, since the main idea around Yams motive is to protect people. All of this is optional because hey, you can forge your own story in the RP so all this back story shit is SUPER important, just my opinion. All it does is add a little more depth to your OC.
Now I have t figure out what this person would look like. I personally don't see this part being super important as long as it's not comical. For a strong, knight/guard like man I can't have him being a child, or a dwarf, or some nerdy kid. It needed to fit the persona. So I went in with a few things in mind. They had to be tall, at least average build, sharp features, professional, prepared. So most of those are easy to emulate such as height but what about "Prepared" and other less obvious things? Well, I put that into his clothing by giving him a little armor. With the little armor, it represents himself a bit more. A visual symbol of his reserved nature, and keeping his emotions guarded. Really though, this shit doesn't have to be super deep. Just a general look is fine that fits your character.
And with that, you've got a pretty good looking OC there. Of course your OC may evolve as things happen in the RP (mine sure did!) and they may change but that's all the fun! So don't feel restricted in what you set out in the beginning! But in my opinion, if you do stick to their base traits it makes for a more polished OC in the end.
For some extra fun, you can try exploring some "Story Theme's" with your OC. I didn't plan this during Yama's creation, but far into the RP after I was comfortable with who he was. What do I mean by "story theme"? Well.. I can't find a way to explain it right now but it's kind of like the theme of your OC's story. As it progresses through the RP that is, not backstory. I decided to base Yama's story on the quote “Beware that, when fighting monsters, you yourself do not become a monster... for when you gaze long into the abyss. The abyss gazes also into you.” so I started steering Yama in a direction that would make him more and more desperate to succeed in defeating evil and injustice. I made Yamas story about fighting so hard against evil that he lost sight of what was right and became a monster himself. Which all still ties into something he'd do given his stubborn nature.
Anyway, that's it! Hope this was helpful! Feel free to ask questions or whatever
-Yam