Yay characters. The building blocks of a story. A story isn't a story without characters. A boring character means a boring story. An exciting character means an exciting story.

I know some people reading this may be fanfic writers, and in that case some people can get complacent, just cause it's not original characters, but no. Characterisation is still important. Especially in the case of writing about real people. Real people are complex. Real people aren't just 2D personalities. Real people have lots of quirks, habits, opinions, and personality traits that makes them them. It's easy to miss these things out when writing fanfics, but this is the kind of stuff that can separate an ok fanfic from an amazing fanfic.
I want to just take a little example from one of my favorite fanfics of all time. Probably one of the best ones I've ever read. "Rub his back" by @joonie_b is probably the best fanfic I've ever read, because they take the personalities of the BTS members, and use them to their advantage. They truly take into account each members personality, and that (along with their amazing descriptions) helps you to imagine the story so vividly in your head, as if it were really happening.
Don't take the outside personality for truth, too. I see it all to much, that Yoongi is always represented as the cold one who doesn't want to show his emotions, which is true. He struggles to show love and affection, but I feel like people forget that they've been living together for nearly 7 years now. Yoongi probably wouldn't act that cold or distant from them anymore. That's just an example, but it shows why taking peoples personalities into account is so important when writing fanfics.

As for original works, the struggle is "how do I come up with an original character that is interesting and original, not just the same as every other character ever?" Well, this is something I struggle with a lot too, not that I write a lot of original stuff anymore, although I wish I could. My best advice would be to go off real people. Look at the people around them, what are their personalities like? And don't look at the obvious stuff. That would end up with a boring 2D character. You want to delve into all those complexities of a personality. Maybe get into, or just research personality types, if that helps. It may also help to think of scenarios, related to the plot or not, and think "what would my character do in this situation?" This is good for fanfics too actually. Just spend some time coming up with random scenarios and putting your characters into those situations, that will help you to get a better understanding of your characters and their personality.

And of course, practise helps. The more you do it, the better you'll get. It's as simple as that.