Someone that saves you from yourself. Like your superhero or savior. When you feel like you're just stuck in a hole, it's your wonderwall that saves you. The song "Wonderwall" by Oasis describes one's feelings about their wonderwall. Note that a wonderwall doesn't always have to be a love interest. It could also be a friend, relative, or basically anybody.
Maybe you're gonna be the one that saves me.
But afterall, you're my wonderwall.