Mirage

Casey Jones -- The Turtles right-hand man. Introduced very early in the series, he's a misguided do-gooder, who thinks that the best way for a criminal to learn is by beating the goodness into him. He is the turtles' and April's confidant for life, and surrogate father to Shadow his late-girlfriend's daughter. This is from the City at War story arc.

Renet -- The Mistress of Time. Early in the series, we meet Renet - a ditzy Valley-girl type apprentice to Father Time. She manages to bungle up her assignments and ends up treating the Turtles to an adventure featuring Savanti Romero and Cerebus the Aardvark (Dave Sims). Later in the series, she has matured, and helps destroy Savanti Juliet out for revenge for her husband's death. She appears in other issues as well, and is considered a friend of the TMNT.

Utroms -- The alien species who invented the mutagen and Techno Cosmic Research Institute (TCRI). Disguised as humans most of the time, but the Utroms' true form is a brain-like being which sits in the belly of a cybernetic humanoid body. They inadvertantly created the ooze that transformed the four turtles and Splinter, and the mutant alligator Leatherhead. These beings are mostly benevolent.

Chet -- the young boy who dropped the Turtles into the sewer. Througout the span of the series, we never see Chet but know he is still around due to his tagging of alley walls and other graffitti. In a special called "Fifteen Years Later" by A.C. Farley in the compilation book Turtle Soup #4, we see Chet all grown up, finally getting the peace he deserves after a decade and a half of wondering.

Karai and the Shredder Elite -- Karai is the leader of the Japanese faction of the Foot Clan. She and the best of the best travel to New York to clean up the mess that the New York faction left -- but not without a war, including the turtles. This is from the City at War story arc.

Baxter Stockman

The Fugitoid

Triceratons

Savanti Romero

Gabrielle

Shadow