I wonder if you can go from single player campaign to multiplayer then back to singleplayer? If I was playing singleplayer then wanted to join a multiplayer game to farm exp/help someone out in their adventure then back to my singleplayer adventure, is that possible ? Or do we need to create a separate character for multiplayer games ?
That's not possible at all. When you join a game, you're using the host's second character, with their skills, their level and their inventory. When you leave, that stuff stays with the host.