Many of these lists have been done but why don´t I deserve to make mine! In this list I´m mentioning my favourite NPCs of multiplayer in RDR.
1- Connor Callahan: This guy is the coolest lawman I´ve ever seen. He also seems to be the most intelligent guy in town (that´s what he says). His clothes are great, and also his quotes Am the law and am here to kill everyone, or Judge, Jury, and my favourite, Executioner haha!!!.
2- Kyle Mc Ginty: First of all, you unlock him as a legendary character ( after level 50) so I guess this guy must be someone important. Also I´ve read in this wiki, that he is the missing son of Landon Ricketts, and that´s cool. Also he seems to have a really important place in the Bollard Twin´s gang.
3- The Tudisco Kid: Well, there can´t be more western name than these in this game. Probably inspired in The Sundance Kid, Billy the Kid, and many other legends of the west. Apart from the fact that his character model is the same from Connor Callahan, I found him really cool too. The only difference with Connor is the beard, the rest is the same, but in other clothes. He seems to have an important role in the Bollard Twin´s gang also.
4- Fred Everts: I used to play as this character in multiplayer until level 49 when I got Connor. The thing that caught my attention was that he kinda looks like Rick Grimes from The Walking Dead, when you look him from behind. Also, he is a lawman in Armadillo, and ( SPOILER ALERT ) isn´t send to kill Marston in the final mission.
5- Mother Superior: Yeah !! NUNS WITH GUNS, the coolest thing I´ve ever seen in RDR. She deserves a place in my list.