Roaming is the bane to every attackers existence and a necessary part of defending if you want to be successful. Roaming is what we call it when there are defenders running around the map further away from the objective. A roamers main goal is to annoy and slow down the attacking team so that they have less time to push the objective at the end of the round. Roamers are also usually the defenders getting most of the kills because they are the ones that the attackers run into first. Now that you know what roamers do lets talk about strategy.
There are tons of different ways to be an effective roamer but for any of the to work you need to communicate with your team, communication is key. Talk to your anchors because in the beginning of the round they will be on cameras and they can tell you where the attackers are going to be coming from. There is one strategy that does not require communication and that is when the roamer runs out of the building and tries to kill the attackers right as they spawn. This strategy is extremely risky and you really only see it lower ranks because the attackers will know where you are coming from. It is also kind of lame because you do not even give the attackers a chance to play the game and it is toxic behavior. Another similar strategy is to sit by a door that attackers often enter the building through and kill them as soon as they enter the building. This strategy is not usually successful because more often than not the attackers will be droning out those areas and if you get droned as a roamer you essentially become useless because the attackers will know where you are. As soon as a roamer loses the element of surprise they become much less effective.
Just as communication is very important so is map knowledge. The better a roamer knows the map the more effective they can be because they can get better angles on attackers and maintain their element of surprise. The operators you want to be using to roam are the three speed ones so you can get around the map easiest. So the characters you want to pick will be Ela, Jager, Caviera, Bandit, Pulse, and Vigil. They are all about equal, each one with their own unique abilities that either help your team or make roaming easier for you but you definitely do not want to be picking Rook or Doc to roam.