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.