Karai is very popular with the fans due to the fact that her loyalty to the Shredder/Foot is downplayed compared to most versions of the character, her snarks are hilarious, her design is very attractive, and her Ship Tease with Leo is entertaining (or not).
Xever/Fishface is more popular than his partner Chris Bradford/Dogpound/Rahzar due in part to having a more visually interesting fighting style. It probably helps that the viewers learn far more about him than they've learned about his partner. Come season five, he has also lasted longer than almost all of the other Foot Clan members in the Shredder's inner circle, the others having either defected or died.
Dr. Victor Falco/The Rat King only appears in four episodes of the series, but he's been acclaimed for being the most terrifying version of the character to date, especially thanks to the ever-popular Jeffrey Combs voicing him. The show also got some major points for adapting his dark as hell role in "City at War".
Spike and Murakami thanks to being an endearing background critter and a Cool Old Guy respectively.
Despite starting out as a Base-Breaking Character (mostly due to shipping), Casey has since become a fan favorite.
Despite starting out as divided when fans found out about the character, Slash is now very well liked due to being portrayed as a cunning, genuine threat who provides interesting tension with Raphael. While his Heel–Face Turn was a bit divisive considering it happened a bit quickly, he nevertheless maintains a following who are pleased with the new direction the character is being taken in, especially after he founds the Mighty Mutanimals.
Leatherhead is popular for his design, voice, abilities, and sympathetic backstory.
Ice Cream Kitty has developed a following due to being a genuinely adorable mutant kitty that is made out of Ice Cream and has great interactions with Mikey.
Tiger Claw quickly became a fan-favorite due to being an all around badass bounty hunter who gave Splinter a good fight. He evolved to become the definitive Breakout Villain of the series and easily the most substantial addition to the mythos introduced by the show.
Kraang Sub-Prime is popular for giving the series a Kraang that's more similar to the 1987 Krang, along with being played by Gilbert Gottfried.
Bebop and Rocksteady are popular for being villains from the 1987 series that were re-imagined in the series into more dangerous villains with richer personalities than ever before.
Renet gained a large amount of fans because she is shown to be a capable fighter who is very pretty and adorable to boot.
ProfessorHoneycutt, who has had his fair share of fans since the start of season four.
Ho Chan is well-liked by fans for his voice, design, lines, and powers. Especially since he's an expy of Lo Pan from Big Trouble in Little China, voiced by James Hong himself. He eventually returns in "End Times".
Shinigami, a badass Action Girl who's working with Karai, is popular with the fans due to her design, fighting style, and personality along with how well she and Karai work together.
