Friends and Allies of the Turtles.
- Voiced by: Kat Graham (English), Azul Valadez (Latin American Spanish)
- Action Girl: She tries to be at least. She's not a major powerhouse or has any training but is very proactive and will charge into battle with her bat.
- Badass Normal: She's a normal human but is more than willing to jump into the fray alongside the turtles.
- Batter Up!: Her signature weapon seems to be a baseball bat. She used a mystical one in the first episode and is shown having a bat in her home. If she doesn't have it with her, she'll mostly use whatever blunt weapon is on hand.
- Cool Big Sis: In the novelization, April considers the turtles like her brothers. She's also older than the oldest of the bros, Raph, by one year (at age 16).
- Friendless Background: She doesn't appear to have any friends outside of the turtles and she can be self-conscious about it.
- Genki Girl: She's very energetic and spunky.
- Leeroy Jenkins: Practically rushes headlong into danger in the pilot to rescue Mayhem with no plan whatsoever other then just flail wildly. Her battlecry is even similar to Leeroy's.
- Her Fearless Fool trend extends to her appearance in The Gumbus, where she's laughing her head off while Leo and Mikey screech in fear.
- Nightmare Fetishist: Her absolute glee about ghosts during The Gumbus shows that she has some interesting tastes.
- Race Lift: This incarnation is not a Caucasian redhead, but an African-American.
- Weirdness Magnet: Lampshaded by Leo in "Operation: Normal", where she attempts to have a normal day with a new friend. Of course, it turns out her new friend isn't human.
- Why Do You Keep Changing Jobs?: She never has the same job twice. Became a rather critical plot point in 'Shadow of Evil' where she accidentally finds herself working at an evil shoestore.
- All Animals Are Dogs: Inverted. While Donatello calls him a "dog-thing", he looks and acts more like a cat.
- Cool Pet: He's adopted by April.
- Cute Kitten: Well, cute kitten-ish creature, at any rate.
- Meaningful Name: He gets his name for the chaos that he created in his debut.
- Teleport Spam: His main ability.
- Ridiculously Cute Critter: It's not clear what it is other than adorable.
- Voiced by: Greg Cipes
Also known as just Shelldon. Donnie's personal robot and helper.
- Artificial Intelligence: Basically this.
- A.I. Is a Crapshoot: Starts out pretty nice and helpful to the turtles, but Donnie purposely programmed Shelldon to favor him over his brothers. When Leo, Raph, and Mikey mess with his programming, it reverses and favors the other turtles over Donnie instead, but Shelldon decides that he wants to replace Don instead of just favoring the others. Shelldon's second incarnation averts this and is shown to be incredibly helpful.
- Cute Machines: While not as adorable as most examples, Shelldon sports adorable round eyes and his second incarnation has his mouth upturn in what looks like a smile. His second incarnation looks like a purple sea turtle.
- Deadpan Snarker: Seems to have adopted some of this attitude, probably partly in due to his creator being the master of this.
- Killer Robot / Robotic Psychopath: After Leo, Raph, and Mikey mess with Shelldon's programming, he tries killing Donnie and replacing him as the turtle's brother. He was fortunately decommissioned and was rebuilt as a much more loyal and helpful companion.
- Robot Buddy: Is one to Donnie.
- Robot Maid: Was one to the turtles and Splinter. It was his original primary purpose when Donatello built him. It is unknown whether or not Shelldon still has this role after being rebuilt and remodeled by Donnie. He still serves somewhat of this purpose at least in Donnie's lab.
- Surfer Dude: Not really one, but he adopts the all too familiar Surfer Accent after Leo, Raph, and Mikey mess with his circuitry. Donnie opts to keep this accent Shelldon's second model, surprisingly enough.
- Voiced by: Dave Foley (English), Miguel Ángel Ruiz (Latin American Spanish)
A bellhop turned mutant bull/dance aficionado.
- A Load of Bull: He was turned into a humanoid bull, akin to a minotaur.
- Crouching Moron, Hidden Badass: Despite his clumsiness and lack of skill, he successfully infiltrates Big Mama's party and almost finishes the turtles mission on his own.
- I Know Mortal Kombat: He learns to dance well enough to impress Big Mama by playing the turtles' DDR game.
- The Klutz: Basically the epitome of this.
- The Thing That Would Not Leave: The turtles originally planned an appreciation day to make up for turning him into mutant. This stretches across two weeks.
- Was Once a Man: Went from a skinny human bellhop into a muscular bull-man after being mutated.
- Voiced by: Eric Bauza
A doorman for a pizzeria, who occasionally meets the Turtles.
- A Day in the Limelight: He plays a prominent role and gets a lot of backstory in "Portal Jacked!", with Leonardo being the Deuteragonist of that particular episode.
- Dem Bones: He's a sharply dressed living skeleton. He sometimes uses his left arm as a cane.
- Hero of Another Story: "Portal Jacked!" shows that he has a troubled history with his pirate brother (who dragged him into his crimes).
- Voiced by: Rob Paulsen
- Affectionate Parody: To Frankenstein if it wasn't obvious (also Sloth from The Goonies in term of personality and technically was a member of the bad guys).
- Gentle Giant: He's rather gentle-natured despite his predisposition toward fighting, and is quick to forgive the Turtles despite Raphael hurting his feelings and Leonardo, Donatello, and Michelangelo's prior intent to kill him.
- Good Costume Switch: A fairly mild case, but once Leonardo, Michelangelo, and Donatello start defending him, Michelangelo rubs off the Foot Clan symbol from Frankenfoot's face and draws this series' incarnation of the Turtles' insignia on him, cementing him as one of their allies.
- Logical Weakness: He was put back together by Raphael, and thus, very easily falls apart even from rather weak blows, which results in him having to constantly be put back together. That being said, he does seem to have the ability to fix himself with some sort of magic, but doesn't know how to properly use it.
- Suddenly Voiced: He's notably the first Origami ninja to show the ability to speak, even if in a rather limited, rudimentary way.