I think one thing is giving them multi-layered personalities (though this usually only becomes visible the more often they show up in different contexts imo), and for character development you need to give them a Want and a Need, ideally the two create a conflict so the character needs to work on a solution within themselves.
My MC Endres for example has the deeply settled need for love, appreciation, and closeness. However, whenever he is close to someone, it turns out badly and he doesn't want anyone to get hurt anymore. He also craves for affection from his love interest but knows (or believes to know) that his love interest wouldn't be happy with him, and he wants to see him happy and safe above everything else.
Throughout different lessons he learns that 1) he isn't at fault for wanting affection, 2) he isn't at fault for others getting hurt by their own decisions, and 3) sometimes people like you back even if you don't expect them to and genuinely are happy with you around.
However, Endres' struggle isn't about whether or not he wants or needs to survive for the greater good because he wants to serve his country the best he can and everyone confirms he can do so the best in surviving. So here won't be a really good chance to show character development.
His sister Malika wants to be perceived as independent and powerful but she needs to ask for help for her emotional/psychological well-being. She does not struggle with the want to fulfill her political role as she can still ask for help despite that.
Mae, yet another MC, is prone to jump into action because she feels restless if she doesn't do anything, she wants to change her negative situation actively - but she needs to be patient for her girlfriend to open up to her, she can't improve their relationship completely on her own as, well, it always needs two to tango.
I think you get my point now.
Another thing is, as a lot pointed out before, that they should stay true to their character in several instances. A truly humble character wouldn't boast about their success, and neither would they dress in absurdly expensive and flashy robes. With their body language they probably wouldn't want to stand out much either and might even be overlooked in a crowd of people.
If characters talk about how nice/selfless/humble/... they are, they usually appear to be the exact opposite. I realised this while I was reading Dickin's Bleak House - one of the main characters always talks how humble and virtuos she is, but truth be told, that only makes her look very vain and gives her a very big "holier than thou" attitude, not exactly traits that make her very amiable.
A "show don't tell" attitude is always the best imo when it comes to character traits - if a character should be perceived as especially nice, show how they donate to the poor anonymously/in a "don't mention it" way, if someone is bullied, they'd stand up for them, or ask for an extra lunch package when they see their classmate's family can't afford their kids' meals anymore.
About character flaws, I'd be a bit careful with "don't use this as character flaw because it actually isn't one" advice since I think a real flaw is something that literally sabotages the character's ambitions.
Clumsiness can be a ledgit character flaw if your character is required to be not clumsy, for example as a professional thief, but it won't work as a flaw if they work as e.g. a graphic designer because it doesn't sabotage them in the same way. This way, it's just some cute quirk but it doesn't put them into maybe even lethal situations like with the former profession.
Is he a secret agent and literally needs to be closed off to keep national security up? The worst flaw he could have would being a total blabbermouth. But remember to give them positive traits as well that kind of smooth it out, in this case he might be a valuable asset as he can manipulate people well enough that they willingly share their secrets, otherwise why would he be a secret agent in first place?
Is your character's goal to find the love of their life and be married happy ever after? Wouldn't it be a shame if they literally sucked at flirting and can't be honest about their feelings? I think you get the twist.
Also, relateable characters don't necessarily have relateable traits but more often than not relateable struggles. E.g. I don't relate to Keith from Voltron: Legendary Defender because he's a hothead and doesn't think things through but because he pushes people away before he is attached to them and thus has several emotional walls up as he's scared to get hurt yet again. People relate to Saitama in One-Punch Man because nobody recognises his value to society despite being highly qualified - not because they can smash any opponent within only one punch, or went into a similar fitness craze.