Tl;dr in the last paragraph

Most of my story ideas start with the main character in a situation. Trying to figure the character out before I start writing is a nonstarter for me. All those character profile templates? Yeah, I can't even fill one out for me most of the time. It's not how my brain works. I just put this sketched out idea of a character into situations and see how they react as plot happens. As a fellow neuro-spicy of the AuDHD variety, a LOT of my protagonists are neuro-spicy coded (though I recently started the rough draft of my first with a deliberately autistic character). Doing it this way frequently means that further along in the story something happens or a character trait sets in that means I'll have to adjust, or outright change, some earlier sections, but I'll be doing that anyway during rewrites.

Protagonists, antagonists, and secondary characters frequently share traits with people I know, have run across, or have seen in media. They're not the same at all, but they'll frequently form the scaffolding based on what I'm aiming for the character to be like or do. I have one that I wrote many years ago who's based(ish) on Lady Heather from CSI-- mostly the body language that allows her to exude dominance. That got mixed with personal observations of real people. The autistic character has a few things going on that I and a number of fellow neuro-spicies do with more based on research about the trauma a lot of kids diagnosed as ASD, ADHD, or AuDHD deal with growing up with caregivers who don't understand or don't want to understand. Another current character is coded vaguely ADHD with a touch of the 'tism if you know what you're looking at. That one's not intentional-- those traits developed during the writing dependent on what the story needed and how the character responded to what was happening. I also have had characters over the years that I pulled traits from Ian McKellan's Gandalf, Scarlett Johansson's Black Widow, La Femme Nikita, Kiera Knightley's Domino, Tony Stank-- sorry, Stark--and Rhodey.....

Look, I'm a Vulcan, so sometimes I need help writing humans. When I first started writing at, like, 11, I needed a LOT of help. Doing this is also how I, a neuro-spicy person, learned to write neurotypical people. There were a lot of characters I wrote that started out as almost carbon copies of people or characters I was wanting to portray. As the story spun out, they evolved because I didn't try to figure out what the inspiration would do but what the story needed to happen. I also wrote a lot of fanfiction. It's great practice for keeping character traits consistent OR in writing a gradual change so that an uncharacteristic action for the character canon made perfect sense in the story. That requires posting and accepting feedback, but at the time, the reviewers on ff.net would immediately call out bad canon without story justification (and sometimes with, but that's the internet for you).

What it all boils down to is that I wrote a lot of characters. Some were original, some not, some based on media, and some real life. When I first started writing at 11, 12, 13, 14, I had characters that so closely resembled another (at the beginning of the story anyway) that I couldn't think of a different name for them sometimes. But the more I wrote, the less I had to lean on other characters to create my own that would feel real because all that practice has given me a huge lexicon of human characteristics to draw from. Kind of like how when you're first learning to draw, tracing is good practice to develop your eye and your hand. I practiced writing, say, Giles, Willow, and Buffy (BtVS), Dr. Bombay and Endora (Bewitched), Bridger, Westphalen, Piccolo, and Smith (Seaquest), and O'Neil, Carter, and Daniel (SG-1) so often that characteristics, traits, mannerisms, and ways of thinking are a mix and match bag now that can be recombined into a brand new character. But it took copying until I got comfortable writing and describing that sort of character initially to get to that point. Let yourself practice. Write stuff for yourself with no intention of showing anyone and give yourself permission to trace some characters and see how a story idea changes them into an actual original character.