>>36601973Grimsley was brought back for 3 reasons:
1) As a Gen V character, he serves to reward SM players that have played all of the games throughout the past 20 years.
2) He gives you the Sharpedo Jet. Since we already had a Water-type specialist give you the Lapras PokeRide, we needed a Dark-type specialist to give us the Sharpedo Jet. Nanu wasn't introduced at this point, so Grimsley served that purpose. (Grimsley was chosen over Sidney and Karin because his games take place at a much later point chronologically, so if he had to be redesigned it wouldn't be as much of a major change in age.)
3) The games wanted to give us a Dark-type specialist that actually served a purpose in the post-game. Nanu didn't do jackshit after the Ultra Beast post-game, and his character as a lazy, selfish person made him unlikely to pursue battling for the sake of it. Grimsley is different; he loves battling because it is much like gambling. Grimsley served as a good boss and Dark-type specialist for Battle Tree. (Game Freak divided type specialists as either Title Defense challengers or Battle Tree opponents.)
