The only additional party members you can have are the summoning dolls (you can get one per act). They have a limited range, which can be upgraded, but are at least useful as mules and to trigger traps.
There are quests in the 'cities'. In the last 2 levels of act 1 there are a couple 'find the blank' quests as well as another more elaborate quest. The imp village in act 2 has a few quests that can be solved within the village itself, as well as some which require exploring the surrounding areas (there are a few friendly NPCs to meet there, but no other settlements). In act 3 there is a temple complex which is larger and has more / more complex quests than the imp village. There are a couple other places with friendly NPCs where you can get quests, but they are fairly small. Both acts 2 and 3 feature a variety of scenery. Act 4 is within the Raanaar academy, where you become a riftrunner, and has more quests / riddles and exploration, and less hack-and-slash.
You will be unbound from the DK near the end of the game, so will be alone in the after game battlefields (unless you use the summoning dolls).
If you give the DK a bow / crossbow and set him to aggressive (right click on character portrait), he will require less management. If you click and hold the mouse button down to direct them, your characters will keep their relative positions; when just clicking, the trailing character may move alongside or ahead of the leader.
I had my DK use a sword and then 2 handed sword. My hero used a club or fighting stick most of act 1 (quite effective against skeletons), then started using a crossbow. Playing again, I think I would have the DK use a bow and the hero be the warrior (maybe sword and shield).
