Characters who are important enough to interact with your main character regularly need to be multi-dimensional, not flat.
What is a flat character?
One that only has one set of attributes, who always has the same kind of emotional response to situation. Check the words you use to attribute a character’s speech; if s/he is always sneering or whining or laughing, then you might have a problem. Multi-dimensional characters have different facets to their character. Even the bad guys have good moments, and the good guys can be jerks sometimes. What is interesting are the circumstances that make a person act slightly out of character.
Unreliable teen narrators (like Melinda in SPEAK) make this harder on the author, especially when writing in the 1st person POV. The narrator is still maturing and has a limited scope and understanding of the world. It is helpful to craft a few scenes where the reader can assess more about the situation than the narrator does.
If your character is a chord instead of a single note, your story becomes richer.