Dream is an INFJ, perhaps even an ISFJ, IMO. I just get that vibe from him. He's very strongly bound by his duties and responsibilities -- often represented as a strong J trait, and almost never as a P trait, if you'd like to play along with the stereotypes, and you unfortunately have to if you're really into classifying fictional characters. And a stern taskmaster, to boot. I also don't see his personal values as one of his primary motivators at all. He can be vehement in defending his own perspective on things, or disagreeing with those that don't suit his views, but certainly (as can be seen from a great deal of Death-Dream dialogue) he'll take others' views into consideration, and even concede that they are more right than he is.
Also, as seen in Season of Mists and Brief Lives, he can be very accomodating towards others and is well acquainted with social structures and gestures. He offers his hospitality to every planar visitor that comes to him in the former series, converses with each of them in private, and in the latter one genuinely attends to his sister Delirium's needs when she comes with an offer to find their brother, even though he's in great emotional distress at the time -- going as far as to confide in his servant that he only goes on the adventure to lessen said distress, which, again if you're into the stereotypes, might be an indicator of "manipulative and controlling" Fe.
You could even make an argument for his tertiary Ti at work when you consider his outsmarting of the demon -- whose name I forgot -- in the first series, where he wins a verbal sparring match with the phrase "I am hope." and leaves the demon flabbergasted, after a period of analysis where he figures out how the demon plays the game and then responds accordingly.
Though I should note that in Dream's case just about every suggestion should be taken with a grain of salt -- what with being nigh omnipotent and all.
