Do you remember this specific tip?
Give a fault to every character and gradually make it disappear (with reason)
Don't make your characters mary sues. Give all of your characters meaningful faults and make sure you step-by-step correct it with a bond (friendship or love)
What if I said we love Marvel because of that?
Yeah. Closely observe every Marvel character, and you'll see why. Each Marvel character has their own flaws. But they gradually decrease it over time.
For an example - Tony Stark has high Anxiety, guilt, perfectionism and pessimism. Everything he does mostly turns out to be a failure. Some of us have anxiety, some of us have survivor's guilt, some of us have perfectionism and pessimism. Over all, Tony Starks has flaws that we all can relate to. I personally relate with him because of my perfectionism, pessimism and anxiety. I have this fear that everybody depends on me too. This is why I love Tony Stark as a character. His personality talks to our hearts, our emotions, our fears. His character development is slow. That's exactly how you build character development. And, in the end, nothing ever satisfies him for his guilt but his own sacrifice to save the universe.
We have Gamora, Thanos and even Wanda. All because we love their struggle and trauma and how they develop through them. Peter Parker, who is always willing to sacrifice himself for others' well being. A people pleaser.
This is why Marvel survived so far as an universe. It seemed to me that they also wanted to include guardians of galaxy to avengers but they seemed not too useful ? no offense, but they quickly avoided that by making Thanos and Gamora side into the guardians of galaxy. There is a villain side, there is a hero side. That's how you make a story. They have evaded plot holes by combination. You should take Marvel as an example and build character with their flaws with addition to your own personal experience. Why their trauma feels so real? Because the writers must have gone through what their characters had.