From my perspective, Duke would be perfect for Dawn, as he would treat her with respect (which even Buffy would be impressed by), and even be a good listener if she wanted to talk.

Plus, if he was an ethical vampire, he would team up with Yami Yugi in order to take down the criminals and, if they were wealthy, drain them of their blood and their bank accounts before giving the money to Dawn and Buffy as their way of assisting the Summers sisters in paying the bills and solving the money problem (which began after Buffy was brought back to life in the episode Bargaining, Part 1 and lasted through all of Season 6, as I recall).