- Played by Isabela Moner
CJ is the series protagonist. Fearing the loss of her best friends she sets out to do 100 things with them before high school no matter what wacky situations they encounter.
- Ambiguously Brown: There are a few moments that hint that she may be half Latina. Her actress is of Peruvian descent.
- Creepy Doll: CJ's doll Angelica freaks out her dad.
- "Do It Yourself" Theme Tune: Her actress sings the intro.
- Egg Sitting: Her mother gave her a flour baby to prove she's responsible enough to have a guinea pig.
- Only Known by Their Nickname: Her full name is Caroline June Martin but everyone, even adults call her CJ.
- Expy: Of Ned Bigby.
- Stuffed into a Locker: Happens in "Raise Your Hand Thing."
- The Smurfette Principle: The only girl in her group.
- Played by Owen Joyner
The "cool guy" of the group who became CJ's friend after she saved him from choking on a stuffed horse in kindergarten.
- Played by Jaheem Coombs
CJ's brainy buddy with several fears and quirks.
- Played by Jack De Sena
The school's guidance counselor who often helps the three friends with their bucket list. Because of this, he is constantly on the verge of being fired.
- Played by Brady Reiter)
Her Highness, Mindy Minus, the most popular girl in Pootatuck Middle School
- Abhorrent Admirer: Towards Crispo.
- Alpha Bitch: She's treated like an actual queen and her authority is even more domineering than the principal.
- Brilliant, but Lazy: Mindy is arguqbly the smartest and academically capable. Unfortunately, she never shows off her true potential. As shown in "Be a Mad Scientist Thing", she preferred to have college students build her a robot for the science fair even though she could've easily done the project herself.
- Everyone Loves Blondes: Just by looking at her, you can probably guess how many girls wanted to dress up as her for Halloween. Okay not really, but just look at her. She gets surrounded by boys on a regular basis thanks to her unparalled beauty.
- Jerk with a Heart of Gold: Despite her brattiness, Mindy tends to have a few Pet the Dog moments. In "Sit at a Different Lunch Table Thing!", she appears to be mad at CJ for dropping her mother's ring down the sink, only to find out that she had swapped it out with her own ring to make Hader believe that it was lost for good. At first, it seems that Mindy is going to act like an Ungrateful Bastard, until she thanks CJ and gives her a hug.
- Modern Major General: You'd think that for someone who spends so much time on their appearance wouldn't care or even time for school. Surprisingly enough, Mindy is actually a top-tier student.
- Played by Lisa Arch
The new principal of Pootatuck Middle School. She is very narcissistic and hardly cares about anyone but herself.
- Dean Bitterman: She is in charge of the entire school and often abuses her power to get what she wants.
- It's All About Me: She has a massive ego and praises herself all the time.
- Jerkass: Her authority causes her to act like a jerk whenever she desires, like when she stole a ring belonging to Mindy's mother in "Sit at a Different Lunch Table Thing!".
- Meaningful Name: Hader sounds like "hated", which is how everyone feels about her.
- This Is Not a Drill: Tries to invoke this in "Find Your Super Power Thing!", where she sets off the fire drills and wastes lots of fire extinguishers to have the record for the fastest fire drill. Eventually, the teachers begin to get annoyed with the constant false alarms, so she deliberately attempts to set a real fire to force them to leave, but it ends up getting out of her control.