TMNT 80s intro, who is Jack?

[Maxmillian 26]

Exactly who is Jack that they are referring to in the 1980s TMNT intro.

"That's a fact Jack"

Is it Jack from MGS games.

Is it Jack the ripper

Is it Jazz Jackrabbit

Is it Michael Jackson

 

Who is it?

[Samurai Equine 51,559]

It's no one, but also everyone and anyone.

"That's a fact, jack" was just a common slang phrase from the day. If someone said it to you, then you are Jack. The same as calling someone bub/dude/buddy/etc.
Pretty sure people only said it because it rhymed. There was usually some kind of implied hostility when people said it, but you could also use that phrase playfully as well. 

How do I know? I was born in the 80's. I was there.