Hey Matt! If you have the Pixelmon mod (you could use the TechnicLauncher to easily download a modpack for Pixelmon with a simple click) you can use the 'Chisel' item in the creative menu to add NPCs. The NPCs can be given a wide variety of skins to closely match the characters of the actual FireRed/LeafGreen. For important characters such as Prof. Oak or Blue, you could use a player tracker to find out people who currently use those skins.
I hope this helped in some way! :D