Well, Ash was invented for the anime. The original person was some random dude named Red. Gary's design/animation model was pretty much taken from the character Blue, though.
Also, Pikachu actually follows you around and shit in Pokemon Yellow. You can interact with him and stuff. Wish they'd expanded on that, but they never really did. You can set a Pokémon to follow you in the remakes of Gold and Silver on the DS(HeartGold and SoulSilver, respectively), but they don't do much otherwise.