IDK about Reptile.
I feel as though Reptile is only popular because of aesthetics: He is one of the first ninjas, with cool powers and a popular colour. His background story and importance is irrelevant to his popularity. He has never been relevant to the plot in any game and could be replaced by basically any character, which is extremely unfortunate. I think if NRS handled the games, especially the new generation games, more intelligently, we could have some interesting characters to become attached to.
Mainly, giving each character all of the following: A. A storyline and POV (See: Game of Thrones) B. Personality and proper marketing (Loved the vignettes from last generation, but most of them were unused or pointless. Characters like Rain, Scarlet, Mileena were all given great support prior to the game, only for them to be irrelevant in-game. Worse, NRS likely has no interest in developing some of these characters further.)
I like Reptile, but I literally cannot say a single thing he's done in all his games except like get tricked by Nitara to become Onaga, which in turn was irrelevant because he was in Armageddon. :/