Writing up a new character is like meeting a prospective romance online.
You see all the important information up front: eye-color, activity preferences, religious beliefs, birthplace, occupation, general sexiness-level, etc.
But until you play mini-golf or storm the keep of a Dread Knight together, you have no idea if he/she is a soul-mate or a dud.
Besides, as Dumbledore said, "It is our choices, Harry, that show what we truly are." How can I know who my new character is until he's had a choice to return the princess to her father or tear off her head and wear it as a hat?