Theodora 'Teddy' Rebecca Duncan is the second oldest child of the Duncan children. She makes video diaries to guide her little sister Charlie through life. Her parents are Amy and Bob Duncan. Teddy's siblings besides Charlie are Gabe, PJ, and Toby Duncan. Teddy is portrayed by Bridgit Mendler.
Teddy is a kind, intelligent, creative, responsible, 19-year-old girl who cares about her family and friends. Teddy loves to go shopping, hang out with her best friend, Ivy, and go to the movies. It is suggested that her favorite colors are blue and purple. Teddy gets excellent grades and excels in English, math, and science ("Teddy's Little Helper"). She also likes music. She was the Beast in Beauty and the Beast, and she was in her school's fourth grade production of The Sound Of Music ("Sleepless in Denver"). It is unknown which part she plays, but Bob finds a flier to her production ("Charlie in Charge"). Her best friend Ivy calls her "T". Teddy can get easily distracted, especially if it comes to her high school crush, Spencer. However, Teddy is sometimes shown to be jealous or angry. She got angry at PJ for bringing home the wrong baby from the park ("Baby Come Back"). Teddy gets jealous when her best friend Ivy becomes "textmates" with her mother (The Curious Case of Mr. Dabney). She sometimes hides secrets from her parents in a few episodes ("The Curious Case of Mr. Dabney"). Teddy also is easily embarrassed, especially by her family members. Sister of PJ, Gabe, Charlie, and Toby.
She blushes when Spencer's rich parents see her family members acting ridiculously on the local news. After admitting being related to them, she actually burps out loud twice and can't deny who she is and confesses to frequently sweating, farting, and burping ("Charlie Goes Viral"). Teddy can also be extremely sensitive about her grades. She gets upset and even a little mad when she gets her first B ("Teddy's Little Helper"), even though Bs aren't that bad. She goes as far as even using Charlie just to make her teacher happy and get an A in Teddy's Little Helper. In "Teddy's Bear", she angrily snaps the point of her teacher's pencil when he refuses to change her grade and ends up getting detention. She also gets mad at her ex-boyfriend Derek when he's not upset that she broke up with him, as she was expecting him to make a big deal out of it and beg her not to break up with him. Her catchphrase is "That's so unfair!"