Rainbow is Phineas' twin. She lived a happy life with her family - until her eighth birthday. At eight, she was kidnapped by Heinz and shot with the Serve-Heinz-Doofenshmirtz-inator. That's pretty self- explanatory. When she was ten, Phineas began searching for her, eventually finding her. Love prevailed - sappy cliché, I know - and she remembered Phineas.