The one most commonly seen onscreen is a tabby cat with brown stripes, white snout & stomach along with other areas of white, what appears to be a large rectangular bald spot on it's back, green or yellow eyes and many tears & scars. However, other cats are seen occasionally as well.
Rise of the Turtles: A kitten is shown to be sitting on a dumpster when Leo looks into an alley that "could have an adventure". It is last seen running off as the turtles continue exploring.
New Friend, Old Enemy: The kitten reappears. When the Foot soldiers debut, they are stalking the turtles. However, the latter eventually manage to hear them. They began searching, only to find the kitten. It is apparently no longer a stray, and has an owner who named it Mittens. Mikey tries to be friendly and escort the kitty back to the man's apartment, but obviously just freaks them out.
Panic in the Sewers: Mittens gets replaced by the tabby. After April's pizza delivery trick doesn't work on a Foot soldier, she runs over to an alley nearby to call the turtles. The tabby was sleeping on a trash can next to where April stands at, and it immediately wakes up to meow at her. It stares at her as she tells the turtles that she has another idea, then meows at her again when she discards the pizza and walks to the apartments. It meows one last time, once again to April as she struggles to not fall off the rundown church. It is last seen when April returns to the ground for more over-the-phone talking, which Dogpound hears. He turns around, and scares the cat away.
Mutagen Man Unleashed: The tabby makes a cameo in this episode after Leo severs Timothy's acid hand. Acidic blood shoots out of his wrist, hitting the cat in the butt. It meows in pain then runs away.
Of Rats and Men: The tabby actually has a main role in this episode. April was leaving Antonio's pizza restaurant with Casey and Irma when a rat runs by. Needless to say, this startles Casey. Irma criticizes him for his musophobia then the cat also comes onscreen, revealing to be the reason why the rat was running away. April interrupts her chasing of the rat, adoring the cat. Seeing that it is stray, she and Casey take her to the turtle's home. When they carry it into Donnie's lab, Mikey happily takes the cat. However, he accidentally lets it eat some mutagen mixed with ice cream, causing it to mutate into living ice cream. Nevertheless, she becomes the turtle's pet and is allowed to live in their freezer.
The Manhattan Project: There is apparently more than one of these stray tabby cats. A cat just like Ice Cream Kitty before her mutation makes another cameo. It is seen sifting through a trash can and gets scared away by Leo, Raph & Mikey falling through a portal and into a dumpster.
The Invasion: As Mikey looks out the window of April's apartment for another glimpse of the Kraang onslaught, a black cat is seen meowing fearfully in a tree during the zoomout.