Teenage Mutant Ninja Turtles: The Ninja Frontier is the best web American animated series, mainly set in New York City.
Splinter was the pet rat of Hamato Yoshi in Japan who followed the teachings of his owner from his cage. When forced to America and after his master's death, Splinter found himself in the sewers of New York where he found four turtles. One morning Splinter awoke to find the turtles playing in a radiocative ooze spilling out of the steel drum. Soon, he noticed that not only were the turtles changing, so was he. Eventually, he grew to his fully mutated form and began to teach his adoptive sons the ways of honor and the path of the ninja. After venturing to the surface for the first time, the teenage Turtles discover an alien race called Neurotroms is planning to take over New York City with their plans involving the same mutagen that created the Ninja Turtles. The Turtles befriend the teenage April O'Neil when she and her grandfather Larry have been abducted by the Neurotroms alongside some other captive scientists. While Larry remains a prisoner of the Neurotroms, April becomes an ally of the Turtles as they help her to find out where the Neurotroms have taken her grandfather. The Turtles also learn that the Shredder is leading the Foot Clan into tracking down Splinter and his sons to put an end to their clan.
- Teenage Mutant Ninja Turtles - A team of anthropomorphic freshwater turtles who are the protagonists of the series:
- Leonardo - "Leo" is an enthusiastic student, learning how to be an effective leader for his three often unruly brothers. He wears a blue mask and fights with two katana swords called Niten Ryu. Aside from his sensei, he takes leadership references from his favorite sci-fi series Space Heroes which annoys Raph to no end. He also has a crush on April which his brothers all tease him about. He is especially fond of Karai, with whom he entertains a brother-sister relationship.
- Donatello - "Donnie" is in charge of the design and manufacture of all of the tools and weapons in the Turtles' arsenal, and he is also a gifted hacker. He wears a purple mask and fights with a rokushakubo which converts to a naginata. This incarnation of Donatello is depicted with freckles and a gap with braces.
- Raphael - "Raph" often lets his impulsiveness get in the way of rational thinking. He wears a red mask and fights with two sai. This incarnation of Raphael is depicted with a lightning-shaped scar on his left shoulder.
- Michelangelo - "Mikey" loves video games, skateboarding, pranks, comics, horror movies and pizza. He wears an orange mask and fights with two nunchaku, which convert into kusarigama.
- April O'Neil - The 16 year-old granddaughter of Doctor Larry O'Neil who likes to play video games and eat pizza. This incarnation of April O'Neil is Asian American.
- Splinter - Splinter is the mutant rat leader of the entire family, and serves as a father figure for the Turtles. He once was a pet for a renowned martial artist, Hamato Yoshi in Japan. It was from watching Yoshi that Splinter first learned the art of ninjitsu which he then passes on to the Turtles. Splinter often acts as much as a spiritual guide and counselor to the team as he does their teacher. While he does not wield a traditional Japanese weapon, Splinter carries a cane which he can utilize as a weapon (like a bo staff) with fierce proficiency. Unlike the previous animated series, the Turtles often refer to Splinter as their father, rather than their master.
- Foot Clan - A ninja organization that is led by the Shredder.
- Oroku Saki/Shredder - One of the primary antagonists of the series and leader of the Foot Clan.
- Karai - A rebellious teen member of the Foot Clan and an expert kunoichi who stirs up trouble for the Turtles.
- Hun - Shredder's muscle, and leader of the Purple Dragons, an urban street gang that Shredder sometimes utilizes. He constantly seeks the Shredder's approval and leads the Foot into battle against the Turtles. His human form is similar in tone to the 2003 TV series and uses the same voice and personality from 2003 TV series.
- Troy Stonesteel/Lionbrute - A famous martial artist and secretly a member of the Foot Clan, Troy Stonesteel is ordered to New York with Heller to find Splinter. As Stonesteel was previously bitten by a caged lion, the mutagen transforms him into Lionbrute, a horn-backed, lion-like mutant with a large left arm, acute senses, and super-strength. His human form appears to be modeled after famed martial artist and actor Chuck Norris.
- Hisser/Dragonhead - Hisser was an imprisoned Jamaican street thug until Shredder freed him and indoctrinated him into the Foot Clan. Shredder orders this ruthless, accomplished fighter to assist Troy Stonesteel in finding Splinter. As Hisser previously touched his pet green basilisk, Shadow, the mutagen transforms him into the lizard-like mutant Dragonhead.
- Baxter Stockman - Baxter Stockman is an inventor who plotted revenge against a company called TCRI, after he was fired for spilling copier toner.
- Foot Ninjas - The foot soldiers of the Foot Clan.
- Purple Dragons - A street gang that has been extorting shop owners for protection money and have become allies with the Foot Clan. The Purple Dragons were led by Hun and Hisser.
- Khun - The leader of the Purple Dragons who fights with a axe.
- Scar - A member of the Purple Dragons.
- Huto - Member of the Purple Dragons. Huto fights with a sledgehammer.
- Neurotroms - One of the main antagonists of the series. The Neurotroms are a race of brain-shaped aliens from Dimension X that pilot robotic bodies and are responsible for bringing mutagen to Earth. This plot was thwarted by the Turtles.
- Triceratons - The Triceratons are an alien race with the appearance of a humanoid triceratops. The Turtles then become a public enemy of the Triceratons for harboring a fugitive known as the Fugitoid, who possesses a transporting technology that the Triceratons desperately want. The Triceratons wish to use Professor Honeycutt's technology to aid them in their war with a Neurotroms.
- Netherdead - Netherdead is Leatherhead's evil twin brother that was originally owned by a young boy until his parents found out and flushed him down the sewer. The Neurotroms took him to their dimension, mutated him, and experimented on him. After escaping from the Neurotrom base, Netherdead fight against Leatherhead but was defeated and cast out. He is considered dangerous due to his aggressive and sociopathic behavior. His form appears to be modeled after the supervillain Sobek.
- Traag - Traag is a lava-spewing, stone monster in service to the Neurotroms. The Neurotroms bring the stone monster through a portal that connects TCRI to Dimension X. Traag proves a challenge to the Turtles, but Zog ends up dragging Traag through the portal back to Dimension X.
- Slash - Slash is a turtle that was mutated by the Neurotroms. After escaping from the Neurotrom base, Slash ran into the Turtles and attacked them.
- Larry O'Neil - A scientist and the grandfather of April O'Neil. He and April were captured by the Neurotroms.
- Leatherhead - Leatherhead was originally an exotic pet owned by a young boy until his parents found out and flushed him down the sewer, somehow ending up in an Utrom lair. After being exposed to the same mutagen that would eventually change the Turtles, Leatherhead became a massive, humanoid crocodile. He lived with Dr. Lunae and other Utroms whom he considered his family. He was taken by the Neurotroms during their attack, which forced the Utroms to flee and Dr. Lunae was accidentally left behind. The Neurotroms took him to their dimension, and experimented on him. Leatherhead eventually escapes from the Neurotroms, steals the energy source that powers the portal between the Neurotrom dimension and Earth, and retreats to the sewers.
- Utroms - Utroms are a peace-loving alien race that resemble the brain.
- Mortu - Mortu is an Utrom captain and one of the Utroms stranded on Earth during the Neurotroms' attack transporting him to crash in feudal Japan. He remains on Earth though the centuries, later stationed at the Utrom's Tokyo headquarters following World War II. He is saved from muggers by a young Hamato Yoshi and Oroku Saki, who discover his secret during the scuffle. He inducts both into the ranks of the Guardians, and is forced to evacuate the Tokyo facility after Oroku Saki betrays them as the Shredder. In the next year, Mortu later presides over the meeting with Hamato Yoshi, Tang Shen, and the Ancient One, and become their friend as the honored guest. In the present day, he is stationed in New York City in the abandoned warehouse.
- Dr. Lunae - Dr. Lunae is the beautiful Utrom scientist who is Leatherhead's adoptive mother and Mortu's assistant.
- Phoenin - Phoenin is the highest leader of the Utroms on Earth.
- Zog - Zog was one of the Triceratons teleported to Earth mid-battle with the Neurotroms over Honeycutt's teleporter. However, while the other Neurotrom and Triceraton forces are returned home by the Utroms, Zog became lost in the sewers, and was found by the Turtles choking to death from lack of his native air. Believing the Turtles to be Triceraton-allied military leaders, Zog pledges his undying loyalty to them and later "General" Splinter, in particular befriending Michelangelo. Zog participates in the Turtles' assault on the Neurotroms' base, and was a valuable asset thanks to his size and ferocity. Zog even helps them to rescue April O'Neil by attacking the Neurotroms. Zog even helps to fight Traag which ends with Zog dragging Traag back to Dimension X.
- Ancient One - Ancient One is an old ninjutsu master who taught Hamato Yoshi and befriended Mortu as his friend and honored guest.