That was Cheyenne Wright, of Girl Genius fame. There was a (short-lived) time where Elli admitted that she couldn't shoulder it all alone and hired Cheyenne to do the coloring. The comic was arguably better for it, but it didn't last as the two had a falling out after a bit.
As for Complains, he's basically like a stereotypical teenager. Tell him to do something and he'll do the opposite just out of spite, and he thinks he knows better than anyone else in any given scenario. In any decent horror movie, that would make him top of the list of victims, but for all its gruesomeness and body horror, Goblins does have a fair bit of (necessary) plot armor.