Special characters are the random encounters you have with people that are white/blue dots that will randomly appear on your mini map. You have to interact with them to unlock them. There are 14 of these however a specific 12 are the ones that are needed to unlock this trophy.
The following 12 are:Andy Moon
Graham (Vinewood Zombie)
This video has all 12 plus the other irrelevant 2 that are needed for this trophy
This is not my video. Video credit goes to GTA Series Videos