KingOfDust: Solo campaign is only available solo, and the characters are separate. They're completely different things really. The "Quest" mode has you fighting through some areas (chosen randomly from a pool of available areas) that increase in difficulty as you go. No story whatsoever.
The original "campaign" mode has you go through a storyline which varies depending on the class you choose. Honestly co-op wouldn't really make sense for that one, because of the way the story works.
Not being able to use the campaign character is no big deal either. Nox isn't really an RPG. It's almost a pure action game. There's levels but they don't do much, you never assign stats or anything. The only choices you make regarding your character are what class to play and what equipment to use. Well, you can change his skin/hair color and stuff too I guess.
I agree with all of this, a co-op mode wouldn't really work in the solo campaign. If there was one, both players would have to play the same class and that would be pretty dull. Nox is much more of a fast twitch action game than something like Diablo, and I don't think the play mechanics would work well in campaign mode. Still, this is a lot of fun to be had for $5.99. I finished the game twice a long time ago, and I am going to buy it so I can revisit it and finish the game as a conjurer.