It's saying that she'll never be a knight or a prince, or a saint, and if that's whoever she's singing the song to thinks they need then they're wrong. They just need someone to fall back on and if they need her then she'll be there for them but she'll be herself (as opposed to a prince, a saint, a knight, or a kamikaze fighter), helping that person, and being there for them as long as they need her.
It's saying that she'll never be a knight or a prince, or a saint, and if that's whoever she's singing the song to thinks they need then they're wrong. They just need someone to fall back on and if they need her then she'll be there for them but she'll be herself (as opposed to a prince, a saint, a knight, or a kamikaze fighter), helping that person, and being there for them as long as they need her.