Now, I don't know the best advice... but here are some tips I have learned from others. First of all, a backstory helps a lot. You know, how a character gets to where they are today. Any kind of backstory helps flesh your character out. If you don't know how to make a backstory that well, an interesting tip I have learned is to ask the question why. For example, say your character has a certain interest, like, sewing, idk. Well, why do they like sewing? Well, maybe they like being creative and making things, and perhaps sewing is calming for them. That in itself fleshes a character out, but we can make it deeper. I ask other things too, like when your character learned to sew. Well, when she was a kid. But when as a kid did she learn to sew? Well, maybe her mother taught her how to sew, and taught her to be more patient when it comes to that. That fleshes out the relationship between daughter and mother. Also, backstories can bring about some personality traits. When I was talking about the sewing backstory, it mentioned how the mother taught her to be more patient. That is a new personality trait the character has. Now heres a useful way to bring out a person's backstory. Take one of your characters and pick a personality trait that could come from a charatcer's past. For example, if your character is untrusting, why is he/she untrusting? Maybe in the character's past, some people ended up betraying the character, causing the character to be untrusting. Doesn't have to be enormously big, just little things make a big difference. 

As for the flaws thing, that can be hard, but interesting. We dare not make a perfect character, a Mary Sue *shudders*. Here's a tip, never be a Mary Sue, an absolutely perfect character with no flaws. But how to make your character have more flaws? That can be hard. An easy thing to do is to look for relatibilility. Are there any flaws you have? I myself have a temper, and I can be a little  too eager to please. I can apply those to my characters. That makes it a little easier to write their flaws than to put a flaw I have never experienced. When you are writing a character with a relatable flaw that you have, you can think about your struggles with that flaw and apply them. If you have a friend with other flaws or a fictional character with other flaws, you can apply them too. Another tip is to look at your chararcter's personality. Sometimes, if your character has a certain personality, it just screams out a flaw. For example, if you have an energetic and highstrung character, it's probably not going to be very patient. Probably will be a little reckless and impatient. And remember, a small flaw goes a long way.

 Wow, that was a lot. Hope any of this helps! (Sorry about the length admins...)

Well, honestly, you said it. This is the hardest thing to do when writing any kind of story, because there would be no story if there were no characters. 

Flaws can be tricky, but even little quirks help to flesh out a character. Start out small, like if they have a flamboyant character, what do they do when they are bored or nervous? Do they bite their fingernails or lips or blink a lot? Just something to start out with making little quirks. 

As for large flaws, go with what you know. If you know what it’s like to be angry (which is very relatable) give them a temper, or even some anger issues. If you have a friend who knows what it’s like to grow up with a single parent, bring your character to realize that they have a gaping hole in their heart, because they feel they weren’t loved etc (this can result in several flaws or problems, such as anger, depression, shutting people out, or just feeling bitter.) Give your character fears. Fear can range from “I’m afraid of the dark,” to “I’m afraid of failure,” or “I’m afraid of being betrayed again.” These fears could come from their personal experiences or from the plot. Do they have hidden flaws, or is it prominent from first meeting them? 

And Making characters realistic? Think like an actor. How does this person walk, do they stride or strut? Or shuffle or glide. How does your character interact in a group? Do they have a steady gaze with each person, or do they just look at the ground?  All tie into habits, also their personality. This subtly tells the audience or reader, what kind of person the character is, which helps to promote realism in their minds, because not everyone has the same walk or talk. This doesn’t mean they have to be completely unique in their physical mannerisms, but it help you as the author to create a real character with real problems, hopes and feelings. 

I read in a book of writing advice to choose a friend or person you see often, and follow them around and make notes of all their little quirks. Do they say "like" a lot? Do they never put their shoes on all the way? Do they eat their lunch really slowly? Then use those as inspiration for your characters.

Alternatively, you can build your characters by taking a few friends and combining their traits. (You can also just copy one friend directly, but that might make it a bit awkward if you want to do something like, say, kill them off later on.) That way you get realistic behaviors.

One thing I've done is just written about myself under a different name, in a different situation, using my own flaws and character traits. The more I write about the character, the more it diverges from myself, but it's a handy place to start.

I hope this helped! Good luck! 

I'm the worst at creating characters, but I read something once, that said something like "your strengths are your weaknesses and your weaknesses are your strengths." I think it's supposed to mean that there's a flip side to a lot of characteristics, like that if you are a good speaker you may not always be a good listner. Hopefully that helps?

I like to make mt character almost like AEs. But unlike AEs, they're a little more complicated, like two AEs sqished ionto one body. Couple this with someone else's advice (which btw guys has helped me with character ideas a lot), and you have a pretty good character. Or you don't bc I'm terrible at giving advice, but hey at least I tried. ;P

Icarus already brought up the idea of strengths and weaknesses being one and the same, but I wanted to touch on that a little more because that line of thinking has really helped me with character development in the past.

The idea is that when you're coming up with a character, you're not picking out random flaws and strengths and mashing them together. You're finding traits that gel cohesively.

Let's ay your character is funny and clever. A witty person might be likely to also be flippant and insenstive. Or maybe they're incredibly selfless. Kind people might disregard their own well-being in favor of others, or be clingy out of a desire to be needed. Someone charistmatic can come off as insincere, someone brave can be reckless, someone optimistic can be naive.

it follows logically. Most, if not all, "good" traits become flaws in certain situations— or when pushed too far. If you find it easier to think of strengths, start there. Then find the "other side" of that strength— How it can secretly (or not so secretly) actually be a flaw.

Another thing that can be interesting is to think about the people around you like characters. You can think about your friends or family and try to determine their flaws and strengths and see how the connect. You can write this down or just toy around with it in your head. It can be weird to think about, but I think these kinds of examples are more helpful than, say, book characters, because you KNOW that it's realistic because they're, well, real.

I hope these ramblings made sense, lol. Good luck with your writing, Leafy!