If there is one thing a manga artist learns the most it is character development. Fictional writers also learn character development the more they write and work with their characters. Development is not something that happens on a dime, but at the same time it should not take four years to decide that a dry character needs work.
So I will discuss what it is to develop a character and what you can do to get there. Keep in mind there are several different ways of developing your character. These are not rules to follow, but guides only. Do not fully expect them to help you develop your character without any actual work involved. Character development does not work like that.
So what is character development? Character development is a characterization of a character that changes overtime, such as being added on, becoming more in-depth, or applying new behavior because of something that happened in their life. Kiba from Wolf's Rain had some development as initially he was a loner and untrusting, and Rin from Blue Exorcist went from being a sullen and angry teen to becoming more mature and having friends by the middle of the series.
Instantaneouscharacter development does not happen over night. You cannot come up with a character and expect to know everything there is to know about them. The more your writing progresses with the use of your character, the more you will come to know about them.
Look at it this way. Consider that your character is a stranger to you outside of their name. The more you come across them the more you learn their full name, age, where they live, what they enjoy, what they hate, and what their personality is. For example, suppose your character has the performer type personality. Let's suppose your character loves to explore - as mentioned in the performer type personality- and just can't wait to see what that new bike trail is all about.
As you write the first story of this character, the performer type of personality stays the same. This is perfectly acceptable, because in the first story, you're just getting to know the character. At the end of the story, you know more about the character than you did in the beginning of the book, but realize you may not know all about the character.
This is where book two comes in. In book two, it is where character development often comes into play. Remember how the above character in the example loved to explore? Throwing in events and actions into a character's life can shift their natural balance. For example, suppose you had the above character ruthlessly raped while she was on the bike trail. Would her personality remain the same? The answer is no.
Realistically, she would become afraid to go riding her bike on any bike trail, and she would become jittery about going to any place she didn't know. Her exploration desire would plummet. This is the beginning of the character development, though she is not quite there fully. If she were to stay in this behavior, this would only be a partial development and over time would become quite frustrating to the reader. This is where you have other characters to step in to help the character over her tragedy. How she gets through the tragedy is up to you, but remember, no rape victim ever got over their tragedy in days or weeks. Realistically it can take months or years. To gain a better understanding at how healing from such tragedies take place, it is recommended to do research on the subject of rape and to search for those who have shared their story. Keep in mind, not every rape victim will talk or write about their ordeal, even online.
Some people ask, "How does one develop characters so thoroughly?" I can assure you that the proper way is to do it correctly. This means avoiding generators that do it for you. This is not to say all generators are bad, but they do not allow one to become creative in their own right. In general, writing generators do the work for you.
There are many ways to come up with an idea or more without taking someone's suggestion and using it exactly how they implied. For example: Suzy is having trouble coming up with an argument scene between her characters Jay and Emily. She appeals to Ashley for help. Ashley gives several good advice tips such as a disagreement turned argument, one of them did or said something that off set the other, not enough attention or a friend coming between them. Ashley tells Suzy that her characters Dave and Jack, two lovers, get into a major argument because Dave was caught cheating on Jack with one of Jack's friends as an example. However, because Suzy doesn't allow herself to consider all the options and all the reasons people argue and fight, she uses Ashley's example as it was, thus having Emily cheat on Jay with one of Jay's friends. This is basic plagiarism and is heavily frowned upon in the world of writing ethically.
As you can see, writing may seem like it's too difficult, but often only to those who are afraid to do their own research and those who do not want to do the actual work of it. Anyone can write, but it takes dedication and commitment with the desire to learn, and the desire to practice to become in depth with developing characters.
You can check out several good materials here on writing.