Because Pokemon: Let's Go, Eevee is, in more ways than one, a revamped remake of Pokemon Yellow, it was expected that a number of core aspects of the original game would continue to be in place. Since the game continues to occur in the Kanto Region, it is up to your New trainer to stop the infamous Team Rocket. Of course, due to the game being a new version of Pokemon Yellow, this means that the characters Jessie and James leap from the Anime Pokemon Series in order to make another return to the Mainstream Pokemon Games.
Even though it has yet to be revealed how strong their Pokemon will be, leaks have already shown that Jessie and James will continue to have the Koffing Evolution Chain and the Ekans Evolution Chain when in battle. These are the same Pokemon they had back in Pokemon Yellow, and one should be prepared to face these same Pokemon during their encounters with this most iconic tandem of Team Rocket.