I think Jester has already been corrupted by the shapeshifter like the others (how they said that they are hearing voices). The logs we found in his room about the expedition said that he started hearing the voices after interacting with a weird monument on his expedition. Something about him feeling weird and stuff. I'm not sure why he ran away and stayed in the bandit's camp though. Maybe the reason why he wants to be called Jester? Cause he's just casually playing around to see how everything would run.
oh that could be true! this could also give a reason to the trigger for the shapeshifters, that weird monument! and before the others could explain and take him he went with Seth to the camp and still doesn't know! maybe we use this later and get Jester as a shifter on our side? or your idea of him being called Jester cause he's fucking around... either way thats pretty good!