My most influential would be Kashika.

Like foervraengd, I haven't made her comic yet. It's still in the process of being written. However I designed her original concept back when I was about 14 (2004), and bare with my incredibly underdeveloped mindset at the time - her purpose was originally to serve as a villian. Why? 'Cause she was going to be the seductress. Something which was universally considered 'bad' by my peers in highschool and immediate known culture. However as I got to write her character more, I actually began to understand her psychology and perspective. I eventually realized she (and thus, other real world women like her) aren't "bad people", they just see things differently. And in a way, are lot freer for it. So yeh, in a way she made me a bit of a better person.

Old art time!

Vespen was my first anti-villain and taught me that character make-up is not always black and white. He was a Captain and a soldier, ruthless in obeying the law and enforcing it, but over time he began to care for others against his better judgement and eventually went back on everything he believed in when he murdered his fellow soldiers in the defence of his criminal friend. I didn't intend for that to happen, and when this arc began to form it blew my mind. He was the most 'human' character I had written up until that point and his development informed everything else.

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I love this question!
Out of all my OCs, the one I knew the least about when I came up with him was Erik:

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What's so interesting about this to me is that he is now absolutely my most complex, plot-important character and holds the weight of the series evenly along with my other MC (Arielle). Arielle was going to be absolutely alone at first, but it just didn't flow right. Erik completed that and took my script to a whole new level, to where I can bend and twist my story at will and add a massive amount of foreshadowing that was never there before.
And honestly I had no idea what to do with him at first. I had to work off of Arielle's personality and create similarities and differences enough for them to foil each other and carry each other's weight.

My only regret is that since my series is so long, my readers don't hardly know enough about them and their personalities since there was a lot of concept to set up. This is what I had to learn through experience of a "first comic," and luckily it's not entirely unfixable!

It would definitely be one of the characters from the first story I ever tried to write off my own back. I called it The Village of Ebony and I'd still like to do something with it, so I'm not sure if it will keep that name, but it taught me so much about writing, characters, deadlines and how to motivate myself to work. All the characters taught me a lot, and all of them have changed a lot since I began them because of it, so it's pretty hard to pick. If you could call a story an OC I'd pick the story itself!
However I think I'd have to go with the main character, Rebecca. She started off life as an amalgamation of lazy writing and the fact that I had no clue how to write a story. She was introduced as a girl with amnesia, who had no memory of anything (even her name) and was called only "The Orphan". This would be fine if it weren't for the fact that I created her like this because I didn't want to have to think about the character at all, and just wanted to start writing. Which of course meant that these were her only defining features and she had no personality to speak of... How I wrote her depended entirely on my mood, and in that way her actions usually made no sense and conflicted with earlier events. Even her age changed as I got older! She taught me how to think characters through much more thoroughly and come up with personalities and backstories before I started writing them.
Because of her, all the other characters I write are much more human (though now that I think about it, only about half of them actually are humans, as such) and I've been able to create characters that I'm happy with and excited to write about! She's still taking shape however, because I find it a lot harder to give a character a personality when you've already been working with them for so long. One day, she'll be perfect though, and I'll be able to describe her in as much detail as I can any of my other characters.
(My favourite from that story probably would probably be someone called Ebonezer - the first person Rebecca meets and a grumpy witch-hunter with weird eyes and the assumption that he's better than everyone else. He also has my favourite backstory from anything ever, and he's probably the character that's taught me the most about physical character design. He's changed so much, but he looks so good...)

This is a quick drawing I did of her recently!

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She's also the first character I ever created so... She's pretty special to me. <3

Interesting question... I think for me it's Mira from Dreamcatchers1.
When I started to draw it, I was only around 15-16, so I was interested in popularity in schools. I never been popular of course, I just tried to be friends with such girls back in primary... I wanted to be someone like them, but I'm different, and deep in my heart I hated that I can't be "normal". So I picked in the story such girls, but made the "strange girl" Sohiru, as the heroine. Mira fells in love with Sohiru's dream being friend (Iko), but Iko is only interested in Sohiru, so I made Mira a foolish person with her useless tryings. But with years I started to like her because of it, and I feel pity about her. I think maybe it doesn't matter how popular or unpopular we are. We all want something that we can't get.

An Iko-Sohiru-Mira scene:

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If we don't count the two dorks I'm drawing a comic about right now1, my most influential characters are probably Talassa and Gale - I can't pick just one of them, since they're both the main characters of their story.

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They're important to me because theirs was the first longer-format comic I ever finished, showing me that I can do this, that I can draw comics properly, that I can tell stories from start to finish. Before them, I almost always got stuck halfway, and found it hard to commit to a story - I always wanted to tinker and change stuff. And while I still do (tinker and change stuff, I mean), I've now gained the ability to push past that and just do.

... Never underestimate the power of finishing things, regardless of quality.

My most influential OC is Princess Tablita Greyward. She won't be appearing in The Chronicles of Oro for a few chapters yet, but I feel that she is crucial to the story in a rather interesting way.
The first time I drew her was nearly ten years ago and also a complete accident. I was doodling in class and 'wham' this new character appeared on my page. Now usually when I'm drawing new characters, like most people, I think up ideas for who they are and what they're like as I go. But Tablita seemed to be the one to tell me who she was and what she was all about right then and there. The problem was, at the time the Twins were royalty and this gal was definitely a princess... So the entire story I had written up to that point was scrapped...
And it got a billion times better. I've been re-writing and editing over the years but the basic structure of what the story is now happened because Tablita demanded to be a part of it.
Since then, there have been a few more major shifts to this project (one of which was reaching the end of the story for the first time and then realizing what the plot actually was about) and each time, Tablita has lead the change. You'd think I'd want to keep her looking like that initial doodle in all her completeness... but as the story and I grew up, I realized that my cast (to put it plainly) was whiter than a box of stone wheat thins. And since this story encompasses the entirety of a world... it just didn't make logical sense. After a few re-designs, I finally came across a look for Tablita that once again, called out to me saying 'this is me, you gotta deal with it' (A la Korra, lol) but now with the clarity of having known her all these years...
I'm really excited to bring her into the story at long last... we'll have to wait for a bit until she arrives but I hope you all will like her as much as I do!
First and most recent designs below:

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Hmm I still haven't made the story fully yet but mine would have to be my OC Bella.

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I made her up for a final school project at the end of high school . We had to make a children's book so I quickly came up with her. But after finishing the project, I came to love her and her friends so I decided to make more stories about them. As a child, she's the most energetic one out of her friends and is at her happiest when her friends are happy. But as she gets older, the happiness fades when she's at risk for losing her friends. I always felt left out whenever I was with a group of friends. Or just a group of people in general. Especially when there is an odd amount of people like 5....3...7 groups of friends. When there were group projects I would be left out. Always. I sorta just got used to it after a while. I would start to leave the group early on before I can experience the situation because I just couldn't bare it! And my friends didn't seem to care all that much honestly? Not like they should/would but I just would always try my best to make sure everyone is included and yet here they are leaving me behind all the time. So Bella is in that situation. She knows that she'll be the one to be left out. She knows that she might even be forgotten most of the time and this story will be just for people that feel that way. For the people that are the one's left behind. ^^b~

I would have to say Jo Amabuoy. He's a cross dresser and very, very insecure about dressing as a boy. As I roleplayed with him more and more, he became able to trust people more, and eventually went out in public dressed as a male. Though I created him without any backstory in mind, he really struck me. You shouldn't be insecure for what other people love about you.
The other character would be Sun Sarah, and she is a sweet, sunny girl who had to take over her grandma's bakery because she had gone into a coma. Sharing the same name and a strong bond with her grandmother, Sun was very burdened. Though she never showed a single ounce of sadness until someone cracked her walls, letting her know it was alright to be sad sometimes. Sun taught me to always give a smile--you never know who might need it.
I'm not adding photos because they are sadly both face claims.