Maybe I am confusing terms here. What are the differences between a Avatar, Origin, Companion, and Henchmen Characters?
If I am remembering correctly, Origin Characters were special pre-built characters you could select from, right? I don't think any one took on of those characters, and I think the player we are talking about was one of the people we could recruit after getting to the initial/starter island.
If it is a problem of "The character that was selected just doesnt have those missions" then that is understandable. I could see just allowing them to get teleported into the same instance as one of the Avatar Characters as a sort of solution. (Right now we are basically starting a stream so the 4th player can at least see what is going on)
