PJ is not very good at studying or at school as much as his sister, Teddy. When she films him for her video diary and says to Charlie, "There's your brother, PJ, doing today's homework at the last minute as usual," He responds, "This isn't today's homework! This is yesterday's homework!" Teddy then retorts into the camera, "There's a chance you two might be in high school together!" ("Study Date"). He also has a tendency to eat a lot of junk food, but this habit temporarily changed by recurring nightmares about him facing the challenges of childhood obesity ("Sleepless in Denver"). He is very passionate about his music. Despite his lack of studying in school, he is mostly very nice to whoever he meets, especially girls. In fact, he describes himself in "Scary had a Little Lamb" by saying, "I don't like to hurt people, that's who I am."
PJ has long, blond hair and often wears jeans and punk-rocker T-shirts, although in some episodes his hair is short. He is pretty immature for his age, also saying," Ooh, pancakes! Yummy num num!" after seeing pancakes on the table. He also loves The Gurgles, a show for 2-year-olds.
Despite being seen and shown as not very bright, PJ has shown many moments of resourcefulness, creativity, and intelligence, implying he has a lack of interest and effort, rather than being really dim-witted. He has also shown a more responsible side to his personality on several occasions, most notably in "Return to Super Adventure Land" where he refuses to continue cooking, as it is causing him to fail school and his dad to put on a lot of weight.
PJ works at Kwikki Chikki. He owns his own band called "PJ and the Vibe" with his best friend, Emmett Heglin. Emmett also worries that someday PJ will end up looking like his dad. He first saw Charlie's first steps when they challenge Emmett and his baby nephew, Mason, to a race in which Charlie starts out crawling but then PJ shows her a picture of "Mommy", and she instantly gets up and takes her first few steps across the finish line, beating Mason and making Emmett mad.
PJ has lived in Denver, Colorado since he was born. In "Charlie's 1!", it is revealed that he sat in the same chairs in the delivery room at the hospital three times, respectively, for the births of Teddy, Gabe, and Charlie. It is also revealed in "PJ in the City" that he is allergic to shellfish.
PJ also likes early nights and doesn't like wearing black ("Scary Had a Little Lamb"). He is also shown to be a very good cook ("Return to Super Adventure Land", "It's a Charlie Duncan Thanksgiving") He was also shown to be very good at reconciling others ("Story Time").
Teddy is PJ's sister. They sometimes hate each other. They kind of misunderstand each other a lot.
Bob is PJ's dad and as such, they love each other very much. Despite this, however, PJ seems to be nervous spending time with him. When Bob asked if PJ is ready to have fun, PJ looks down as he has to go fishing with his dad ("Charlie is 1"). PJ can also be mad at Bob for his birth certificate, where his name is revealed as 'Potty John' ("Can You Keep a Secret?"), or when Bob refers to him as "the nice one" ("Appy Days").
Emmett is PJ's best friend. He also has a crush on Teddy and is convinced that they are in a relationship. Teddy begs to differ. He dated Ivy Wentz for a while but eventually, they broke up. Emmett and PJ also have a band. It is called "PJ And The Vibe".
PJ met Emma at the park. They never kissed, but he got her number, called it, and it was the wrong one. He thought Emma gave him a wrong number, but he actually wrote it wrong ("Baby Come Back").
PJ met Madison while delivering for Kwikki Chikki. He developed a crush on her but saw her flirting with the pizza man and broke up with her because she chose the latter over him because he gave her coupons ("Boys Meet Girls").
PJ met Kayla at the mall while holding Charlie. She turned out to just love Charlie. He broke up with her after discovering that she just was with him for Charlie's cuteness ("Something's Fishy").
When practicing for the battle of the bands, Skyler's and PJ's eyes met and it was love at first sight. They took pictures together in a photo booth and got Teddy mad because her best friend is a girlfriend to him. Both seem they don't get a lot of things. In "Battle of the Bands", Teddy had an idea and gave them a look. He says she only gets that look whenever she has an idea and after that he said that he never gets that look, judging the fact that he's stupid and she agreed that she also doesn't get that look either. They never broke up in the episode, so there is a chance that they will be seen as an official couple in future episodes. In "Teddy's Bear", it was their One-Week-A-Versery. She writes him a poem that doesn't rhyme because she says it was too difficult for her. He gives her a necklace with a piece of churro because that was the last bite of the first snack they shared. Unfortunately, her ex-boyfriend, Brock, kept texting her. During the end of the episode, Brock texted Teddy to go out with him. In the episode "PJ In The City", she moves to New York City because of her father's job. He was heartbroken so he moved there with her. He got a job at Wall Street Subway Station singing and playing his guitar. Unfortunately, he broke up with her because Bob persuaded him to move back to Denver. He said if she really loves him she will return to Denver to be with him.
PJ met Zoey at Kwikki Chikki. When he introduced her to his parents, she was dressed up as her sister for Halloween, who dressed as an average teenage girl every single day. Afterward, she dressed as she normally would. She was actually a goth. He dates her and changes his image to go out with her. But in the end, he tricks them into leaving and then never sees her again ("Scary Had a Little Lamb").
- His birth name was supposed to be Patrick John. He's named after his great grandfather, Patrick John Duncan. Bob messed up his birth certificate the day he was born and wrote Potty John instead of Patrick John. When he tried to fix the mistake, he messed it up again and wrote PP Duncan instead of PJ Duncan ("Can You Keep a Secret?").
- He can put on a realistic stereotypical British accent ("Meet the Parents").
- He can also put on a German accent ("Charlie Shakes It Up!").
- His middle name is named after Darth Vader.
- PJ's blood type is O Negative. (The Jessie/Good Luck Charlie crossover "Good Luck Jessie: NYC Christmas Part 2)