The Fifth Turtles

The Teenage Mutant Ninja Turtles traditionally had four characters. However, there have been fifth members.
Ninjara (from Archie Comics)

The only non-turtle

She is the only fifth member that isn't a turtle. She was a member of Fox People that lived in Japan. She worked for Chien Khan until the Turtles revealed he was evil. As such, she defected to the Turtle's side thus becoming the fifth "Turtle". She had a romantic relationship with Raphael. While she fought with them, she left when her relationship with Raphael caused problems. She went off to live with the humanoid wolf Mokoshan and his Alaskan clan. She had a daughter with him named Moka. Mokoshan was murdered. As such, she went on an investigation to find who killed him.
Slash

Isn't he a handsome fellow?

He was a criminal from Dimension X and ally of Bellybomb and Krang. The trio escaped to Earth via a waste ship, but was defeated by Turtles. Slash was captured after going on a berserk rage. The Mutanimals discovered Slash wanted to live on a peaceful world unlike his world (which was being stripped mined) and took him to an environmental reservation, where he found peace. He joined the Turtles to avenge the death of the Mutanimals by fighting Mr. Null (their murderer). He drove a ship (with Null in it) into the sun.
He has invulnerability and super strength. He is skilled at using the weapons of the TMNT members and "Pantera claws". However, he is prone to berserk rages and short-tempered.
Venus de Milo (from Ninja Turtles: The Next Mutations)

As she appeared on the show

Venus de Milo / Mei Pieh Chi was created for the Ninja Turtles: The Next Mutations. She was a "Shinobi" (their word) mutated turtle. She  met them when she was trying to save her master, Chung I, and eventually joined them. As such, she was a main character. She tended be portrayed ignorant of Western culture. She wasn't trained in Ninjutsu and used magic instead. She was disliked by fans for various reason ranging from being an inaccuracy portrayal of Shinobis and to being a deus ex machina at times. A parody of Venus appeared in the twelve issue of the Image comic TMNT series. She was referenced by the series' on and off again slogan "No girl turtle guaranteed".
Kirby (from Teenage Mutant Ninja Turtles IV concept art) 

Concept art

Despite Teenage Mutant Ninja Turtles III: Turtles in Time being critically panned, plans for a fourth movie, Teenage Mutant Ninja Turtles IV: The Next Mutation (not to be confused with the before-mentioned television series), were on the way. Kevin Eastman (one of the original TMNT creators) drew some concept art. In several of pieces, he drew "Kirby The Fifth Turtle", who was named after Jack Kirby. However, TMNT 4 never made it off the ground. As such, Kirby never had an official appearance.
References:
http://www.comicvine.com/ninjara/4005-51425/
http://www.comicvine.com/slash/4005-51420/
http://turtlepedia.wikia.com/wiki/Venus_de_Milo
http://screenrant.com/tmnt-4-concept-art-kirby/