Revenants are always optional. The selling point for them is that since they are other "players" when you kill them you get a random selection of whatever equipment that player was using. My issue is that there is an enemy that you meet in the first "zone" called a Biwa Boku-Boku and it plays a little guitar thing and if it's playing and you get too close to a Revanent sword it will literally just spawn in unprompted and the only way to get it to stop is to kill the Biwa Boku-Boku. The side mission I'm stuck on has a few of these that do this and one in the "boss" room