Their first appearance is in Chapter 5 - Urban Warfare, when they crash a bus, in what appears to be an attempted ambush and suicide bombing on a group of Zoran Lazarevic's soldiers. This small group are quickly killed by the soldiers. They appear again at the beginning of Chapter 6 - Desperate Times when another small group of them are summarily executed in the plaza by Lazarevic's men. Their final appearance is in Chapter 7 - They're Coming With Us, when Nate sees a small group of them being hunted down by a tank.
Although they are a presence in the game, the resistance fighters are never directly encountered by the player, and Nathan Drake never interacts with them. They fight for a noble cause, attempting to stop Lazarevic from destroying their city, however they fight in vain. They are too few in numbers and far too disorganized to stand against Lazarevic, and their attempted uprising is quickly quashed. Lazarevic's men immediately open fire on any resistance fighters they encounter and mercilessly execute any survivors. Fortunately for them, when Lazarevic's men find out where the passage to Shambhala is hidden, they leave the city with all their vehicles and gear to make a raid there.
- The Resistance Fighters built bridges to cross the rooftops.
- The resistance fighters were originally fighting unmentioned enemies, as Chloe mentions "the city's been on the brink of civil war for years." So, Lazarevic would have really been dealing with the aftermath, and survivors, of the violence.