First person allows deeper insight into the protagonist's character. It allows the reader to identify more fully with the protagonist and to share her world quite intimately. So it suits a story focused on one character's personal journey. However, first person shuts out insights into other characters.
There are technical tricks that may help you create more effective characters. My approach to characterization is not at all technical. I can't really analyze how I do it, but I am sure of one thing. To write convincing characters, you must possess the ability to think yourself into someone else's skin.