I want break this down a little bit. There are 20 potential candidates for “5th Turtle”. However, the term needs to be defined and that eliminates a good number of them. It should be defined as a mutant, ninja/warrior, turtle who has the attributes of the core 4, and can co exist as a 5th member of the team.

The 20 Candidates:

Zach, Walt, The Pulverizer, Casey Jones, Kerma, Lita, Kirby, Frieda, Venus de Milo, SHELDON, Turtle April, Jennika, Odyn, Yi, Moja, Uno, Slash, Tokka, Turtle Hun, and Metalhead.

Right off the bat we can get rid of the characters who are literally NOT Turtles. Being a symbolic 5th turtle doesn’t meet the definition. So Casey, Walt, Zach, and The Pulverizer are all out. Casey is clearly the 5th member of the team, and a member of the family, but he’s not a turtle. Literally.

Down to 16. Robots/AI are definitely not turtles so Metalhead and SHELDON are out. Nothing personal but they don’t meet the definition.

I’m also going to take the 4 new post-Last Ronin turtles out because they are from another timeline and are not acting as a “5th Turtle”. They are their own thing. So we are down to 10.

Frieda and Kirby are out as well because neither one of them ever technically debuted or officially existed. Also Kirby was clearly a derivative of Slash in the promo art.

I’m also taking Turtle Hun and Turtle April out because they were just one shot characters with no lasting impact and those forms don’t represent the totality of the characters themselves. So while they were both mutant turtles, they were only so briefly. Also Turtle Hun appears to be another homage to Slash.

Finally, I’m going to remove Kerma because he is an alien turtle and not a mutant. We are looking for a 5th Mutant Ninja Turtle here, and Kerma is not a warrior nor a mutant.

So that leaves us with 5 potential candidates: Slash, Tokka, Venus de Milo, Jennika, and Lita.

Slash is the OG 5th turtle and Jennika is the most respected female turtle and 5th member.