Kelly Clarkson is a singer who was the first season winner of American Idol. She made a guest appearance in "A Phineas and Ferb Family Christmas", playing herself in animated form. Every time she suggested to sing, Phineas would tell her not to and just relax.
Later, she was teleported to the home of Heinz Doofenshmirtz along with all the extras and the TV stage crew. She would have sung for Doofenshmirtz, but he warned everyone that his machine was about to explode, so she escaped in a hang-glider instead.
After Clay Aiken, Clarkson is the second American Idol finalist (and the only winner) to provide her own voice on the show.
The animated version of Kelly Clarkson visits Phineas and Ferb's Family Christmas Special.