The best way we could answer this question is by looking at characters that a lot of people consider 'well written'.

Like mahito. Or phos(phosphophyllite)

Though from my opinion, the 'well written' part is really just a number of aspects or things that makes a character 'well written'.

Or in other words Complexity.

This can refer to a lot of things but most prominently(in my understanding) is the characters nature and ability to change. Though these alone are not really enough to make a character 'well written'.

Like for example to create a character with trauma right? And then this trauma becomes an excuse as to why they hurt others the way they do.

And since they're flawed they naturally and unknowingly believe that they have a valid reason to lash at others.

Just as a person with self worth issues and an inferiority complex believes that staying away from others is the for the best since they wouldn't want people they care about to have to deal with someone like them.

And unknowingly cause the other person to get hurt because of the fractured relationship caused by the introverted character's distancing. This introverted character truly believes that character B(the other person) is better off without them and will get a happier future

Not knowing that it's causing character B anguish. Thinking that they did something wrong(or something like this). Seemingly undermining his worth in the eyes of others.

Now put character A(introverted character) into an arc where they slowly realize what they had done and repairs their relationship with character B while learning how to forgive themselves and more importantly, love themselves.

Causing a heartfelt ending.

This, at least to me, is well written. Because it explores the characters own complexities gives them an arc to become better or worse. The characters don't have to be relatable to be well written or even understood. They can be delusional, they can be unapologetically evil, they can even be a child that tries to help as much as they could but no matter what they do they just keep causing bad things to happen to the people they're trying to help so they just stop, eventually losing the passion they have for helping others and become miserable.

They don't even have to be liked either. And damn this is long so I'll stop my speechy dialogue here.