As Joy said, Nicholas frequently reminds Cassandra about their "bargain." My understanding is that she was allowed to return to the mortal world in order to fulfill her curse upon Barnabas and drive his soul into the fires of perdition (Great Goddess, I think I'm channelling Rev. Trask!!). Once he brings her back, we presume he hangs around to make sure she gets her job done without further blundering.
One of my favorite scenes is when Nicky and Cass are out on the terrace and he sneers at her for being "so in-COM-pe-tent." I just adore HAA's reading of Nicholas' very fruity dialogue. This stuff is better than heroin--hell, it's better than two sticks of Land-o'Lakes lashed together in a rough-hewn manner!
The Nicholas/Cassandra scenes hint heavily at some pretty dark and gritty backstory between the two, and a lot of fanfiction has been written speculating about just what this was. It's too bad they never did a flashback to show how the two first met.
G.