In Rainbow Six Siege you can play how ever and whoever you want too, but you have to play smart and quickly or else you will end up dying for nothing. Any character can be played as a roamer, but their is two operators that come to mind, where their main purpose is to roam and slow down the attackers. These two operators are Caveira and Vigil, these two operators have abilities that help them cover more ground quickly and unseen which are necessary characteristics of a roamer.
Both these operators have great guns and are really good for roaming and getting information, especially Caveira since she can interrogate attackers. The best playstyle for both of these ops is to get behind the enemy and pick them off one by one or to just waste time so they are on a time crunch to get the plant.
If you play either of these operators you are going to want to wait in or around a spot that attackers usually push from. This could be the ice cream shop when the bomb is in office and initiation on Theme Park or playing in Big tower when the bomb is in game room and kids room on Oregon, this is how you can get a lot of kills and slow them down at the same time since they have to deal with you while trying to push the objective.
You also always need to have a back-up plan incase 3 or 4 of the attackers push from all different angles. This could be a rotate hole or a hatch that is open so you can escape safely and waste their time at the same time. Most of the time you will get pushed by two or even three people when they know where you are, especially if you are in a room with no exits, then you are screwed.
Another thing to keep in mind when playing a roamer is what characters they have been playing and gadgets or drones that are around the map this could be claymores, drones, airjabs, trax stingers, and even people that are holding a angle on their flank. You have to keep all of these things in mind while running around trying to find out how to push them or stall them.
Overall if you want to be a good roamer you are going to want to flank, be patient, be aware, and play smart and quickly or else you will die. Dying also isn’t a bad thing, as long as you did damage and slowed them down, you completed your purpose and wasted a couple of seconds or minutes. You also have to remember that you can play any operator and roam, some are just harder and not as sneaky to move around the map.