You should write your characters with the idea that they will organically evolve through your story.

But for that, you need a strong base that allows you enough material for said evolution.

Make bios of them with elements like their likes and dislikes, their relationships, and other important info. That's a great start.

But I think that the most important things about the character are "What does the character want?" and "What does the character represent?"

If you have the answers to those questions, use them as a compass to guide their development through the story, seeing how those answers clash or complement others.

This is sound advice.

It's hard to say because it really depends on where they start.

A lot of characters come from a basic concept for me. Sometimes it's an idea I want to explore (I want the character to be a burnt out romantic interest in an isekai who's sick of the lead), sometimes that concept is functional (I need a character on the main cast who can bring up uncomfortable subjects), and sometimes it's totally shallow (I love reading this kind of villain! I wanna see if I can pull of that vibe!)

Once I have a core starting point, I can scrap and layer on ideas I want to play with.
Then I start filling in the holes, which is two big tasks, but also my favorite part.

-What kind of person makes all these traits make sense?

For example, if I've decided my character is a shy nerd, but they also joined the knights. Was it family pressure? Do they admire knights and wish they had different skills? Is knighthood the best way to get samples for their studies? Or perhaps they gave up their passion in science to protect somebody they loved. Maybe they just love being a knight, and the books can wait!

-What ways can I fill this that DON'T immediately make sense?

Easy answers can lead to boring characters. Every villain does bad things because they're sad, or because they're Just Evil.
Let's say I decided my knight is a scholar who took up arms to protect their true love. The first images that come to mind are an awkward dork hoping to impress their crush, or a romantic scholar sweeping their fair noble off their feet. Both of those could make sense, but is that what interests me the most?
Perhaps the person they're trying to protect isn't a prince/princess. What if they became a knight to protect a destitute farmer? What could that story be? Are their no knights in their area? Or perhaps their true love, a fellow scholar, was forced to fight on the frontlines, so they chose to join as well.
It's easy to picture the person who would do that as warm and protective, but perhaps they're cold and distant, unable to connect with the person they're risking their life to save. Or perhaps not! Because it's not about being 'easy to understand' or 'unpredictable and unique'. It's about what you like writing, and what feels right for your character.
Other big questions will naturally flow from that. How do they feel about the war? Do they want to confess? If not, why? What were they a scholar of and why did they like that? What made somebody worth following to war? And those questions loop you back to those two core questions, either 'what makes sense?' or 'what makes it interesting?'

Bringing every question back to the same set of traits can feel stale quickly, but I think, once you have a 'what kind of person is this' you can start to figure out how they'd respond to things, and that will fill them out.
If I know my character is 'a cold scholar who gave everything up to silently protect their love, but can't treat them kindly' I can understand a lot about them. They're intellectually curious, willing to go to extremes, either don't understand or doesn't like everyday kindness. When I write them in a scene, different parts of that I've chosen will come to the front based on their values and memories.

I hope that's.... even slightly coherant.

I participate in roleplaying threads to flesh them out, which works quite well for me. It allows them to be forced in situations they might not encounter in canon, and it allows you to know them better.

Downside is of course you have to rp haha, I know it isn't for everybody.

I find that building the story around them helps flesh them out :o Since you need to know where they come from and what their role is

A training exercise for animators is to animate each character from a cast walking into a room and sitting on a chair. Making each character feel distinct in this simple action pushes the animators to think about little things like how their posture and way of walking get across their personality.

In a similar way, I find going over a simple situation, for example "the character just got home and there's water leaking from the ceiling! A cheeky squirrel has sneaked into the house and turned on the bath taps and left them on all day!" and then going through how each character reacts to this, and how it might differ from how you'd react, helps me really nail down what's unique about the character and their place in the cast. Like I know that Rekki is impulsive and angry, so she'd probably get distracted chasing the squirrel until she's reminded of the problem of the damage caused by the leak, while a more levelheaded and non-confrontational character like Subo would call somebody for help and only later try to coax the squirrel out of his house. Some characters might just stand there laughing, others might try to shoot the squirrel etc.

I love making pages where we see multiple characters react to the same thing, like this:

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The mysterious masked vigilante activist who seemed really cool and is a powerful warrior just made a smooth exit... but then promptly started jogging down the street in broad daylight rather than smoke bombing away or leaping onto the rooftops or something cool. How do our other characters react?
Rekki, who has a lot of insecurity issues feels second-hand cringe because she wouldn't want to be seen doing something embarrassing and projects those feelings.
Subo is kind of awed by this complete lack of giving a crap what anyone thinks; to him this is aspirational.
Jules, ever the lover of absurdity, thinks this is both charming and absolutely hilarious, but being used to Neurotypicals finding their big reactions to things strange, covers their mouth out of habit to try to dampen the size of the laugh and "keep it under control". (Like me, Jules is Autistic and sometimes struggles to stop laughing or laughs a bit "too much" compared to what some people would consider "socially appropriate").

There are tons of little ways to get to know your characters. Do they like coffee or tea? Or maybe they hate all hot drinks? Are they a cat person or a dog person? Do they scream when they see a spider or are they the person others would call on to deal with a spider in the bath, and if so do they kill the spider or do they put it in a cup and take it outside? What's their vocabulary like? Do they swear? Do they like to use big, fancy words or prefer to speak very straightforwardly? Think of all these sorts of little things, and they'll start to add up to feeling like a fully fleshed-out person.

I get to know my characters the more I write about them, just like with normal people. All stories are organic. Trying to push yourself with questionnaires and the like will just frustrate you (in my experience). If anything, I like to sketch what I think my characters look like. Having a visual makes them feel a little more alive for me. But other than that, it all comes with the work. The further you follow them on their journey, the more they become real to you.

Seeing some great advice here. A character is designed to serve some purpose within a story, but the most interesting characters are the ones we can imagine in any number of different stories. That's why giving your character the same dilemma another character has faced can help shine a light on what makes them unique.

It's worth noting that the degree to which you need to know your characters is generally based on how much you are looking to set up in advance. If you are playing the long game, with a plan to reveal that a character's parents are actually the evil king and queen the party is fighting against, you will want to know that in advance so you can foreshadow it, as well as understand why they kept it a secret and what they are doing to keep it a secret. If long-term planning isn't your aim, developing them as you write the story can work just fine.You'll just want to keep a design document on your characters and write down all of the relevant information you introduce. ("He wants to be a blacksmith," "sunsets make her sad," "they don't like each other," etc.) Consistency is very important in terms of making a character feel like a real person, and it can be really easy to forget that you made an offhand joke in episode 3 about a character being allergic to milk, only for them to eat pizza in episode 23 with no repercussions.

Also, be wary of "overwriting" your character when trying to understand them. It can be useful as an exercise to fill out character bio templates and flesh out some things you hadn't considered about the character, but if you include all of it in your design document, it can make it harder to write a scene with them since you have so many details to sift through. One way to avoid this is to boil down your character to the relevant bits after fleshing them out as much as you can. Think about how relevant each detail is and consider if it plays into your story. Things like favorite food and hobbies will probably have more relevance than blood type and the fact that they played baseball when they were 7. But then, maybe your story is about a professional baseball player who needs a blood transfusion. It's all about the context.

Most importantly, keep in mind that no character exists in a vacuum. Stories are driven by conflict, and characters are usually created to contrast with the world and characters around them. If you take a character like Batman and drop him into the world of My Little Pony, you'd have to drastically change the tone and possibly even characterization to make the characters and world mesh together. Understanding what your characters value is important because it facilitates putting them in situations that personally challenge them, such as a braggart being humbled or a misanthrope going on a date. Without a surrounding environment to play off, a character in isolation is about as interesting as a single color painted on a white canvas. It may be a pretty color, but the intrigue comes from what you do with it.

In all honesty, I do a little bit of both! I first create a basic diagram of their personalities and backgrounds, and then I bend and warp them depending on the situations I put them in. So that by the end of the story, they're as complex as a real human being...did any of that make sense???