She produces these video diaries at the end of every episode, and at the end of every video diary she says "Good luck Charlie". It was mentioned in "Sun Show: Part 1", that her parents were expecting a boy, hence the generally masculine name "Teddy". Throughout the show Teddy has had numerous boyfriends. for "goody-goody." She and Spencer get back together in "Can You Keep A Secret? In "All Fall Down" Teddy and Spencer break up because they don't want to be in a long-distance relationship while Spencer attends the Boston Institute for Performing Arts (BIPA).
She dated Spencer Walsh in season one, but they broke up after she caught him two-timing both her and a pretty, dim-witted brunette named Skyler, breaking her heart. Teddy shows much care towards her brothers, such as saving PJ from falling out of the treehouse window, or offering to help Gabe deal with Jo, who has given him a black eye. But when Bob's newest employee Beau admits that he likes Teddy, they begin dating. In "Accepted", Teddy finds out that she is on the waiting list for Yale after Victor; later she is accepted.
It focuses on the Duncan family of Denver as they adjust to the births of their fourth and fifth children, Charlotte "Charlie" (Mia Talerico) and Toby (Logan Moreau).
In each episode, Teddy Duncan (Bridgit Mendler) adds to a video diary that contains advice for Charlie about their family and life as a teenager.
In March 2011, a feature-length Christmas Disney Channel Original Movie based on the series, entitled Good Luck Charlie, It's Christmas! When parents Amy (Leigh-Allyn Baker), a nurse, and Bob (Eric Allan Kramer), an exterminator, return to work, they ask their three older children—PJ (Jason Dolley), Teddy (Bridgit Mendler), and Gabe (Bradley Steven Perry)--to help raise their little sister.
At the same time, Teddy, PJ, and Gabe try to deal with school and general social challenges in their lives.
At the end of each video, she (and/or other cast members) says, "Good luck, Charlie" or may even say it indirectly such as "Wish them good luck, Charlie." During the movie, Amy reveals to Teddy that she is pregnant with her fifth child, but this story plot is not brought into the series until the third season, during which Amy gives birth to a baby boy, Toby.
Teddy hopes the videos will provide useful advice for Charlie after they have both grown up and Teddy has moved out.
Dabney, who is often the target of Gabe's pranks and stunts—which is the only time the family doesn't mind them. Both her older brothers, PJ and Gabe, are often seen using Charlie for their own benefit, although PJ shows that he cares about her deeply.
Gabe seems to dislike Charlie the most although, in some episodes, he is seen being quite fond of her. It was revealed in "Charlie is 1", that it was Teddy's idea to nickname her Charlie.
Teddy tries to show Charlie what she might go through when she is older for future reference.
Each video diary ends with Teddy (or another family member, even Charlie) saying the eponymous phrase, "Good luck, Charlie".