To me, a character has to be memorable. They have to have an internal conflict (something personal that they're working through) as well as an external pull to make them rethink the internal conflict. For example, in 'Cameo Appearance,' Cameo was trying to hide from a murderer, but Reggie wanted her to marry him. She didn't want to be in the limelight, but Reggie was a prince, automatically upping the conflict for poor Cameo.
Sometimes, they have a trait that can't be denied. For example, Reggie kept saying, 'most definitely,' because that's what they said on his home island. Cameo, on the other hand, didn't want to be noticed, telling Reggie she'd be fine without his help.
Make the characters realistic, reacting like a normal person to various conflicts. For example, if a tornado's heading toward the character, don't make them decide to make dinner at that point in time. They need to react and get to safety. Or if they're getting an IV, make it hurt. Show how the character wants to beat up the nurse or something.
Your character also needs a 'place' in the story. Are they the protagonist (the good guy), the antagonist (the bad guy), the love interest, or maybe the sidekick? How do they help or hinder the protagonist? That'll help define their characteristics, as well.
Every character needs a goal. Make sure you know that goal for the character before you write the book.
I looked up a few references to indicate other things to consider when writing characters. Here are a few references of note: