What's your definition of f2p ninja?

A ninja you can get by stacking coupons?

In that case naruto ninetails chakra 4 stars, hokage tsunade, hokage minato, ay 4th, sasuke susano'o and hashirama are f2p because all of them appeared more than once in events where you could buy the fragments with coupons or where you had a good chance to get them in a common wheel.

Now, since is blatant this one can't be the criterion to know which ninja can be considered f2p and which one can't be, then lets try to figure a more objective criterion.

If you ask me what could be i would say a ninja that doesn't cost more than two months of hoarding coupons and that doesn't appear as a super rare in a treasure.

Now,since the smart f2p usual income per month is 4500-5000 coupons, then the cap is 9000-10000 coupons value.

Does exist an event where you can get lets say jonin minato with that quantity of coupons for sure?

The reply is yes and is fateful choice. So jonin minato for me is a f2p ninja.

Does exist an event where you can get shisui for sure with that quantity of coupons?

No, in every event so far where you could buy his fragments his cost was around 15000-16000 coupons so by my criterion shisui is not a f2p ninja, but can be considered a low casher ninja, since low cashers get around 2000-2500 more coupons per month than f2p.