Discord will certainly think that Ulquiorra is intersting... at least for a little while. His nihilism and no-nonsense attitude will quickly bore the chaos spirit. It's hard to have fun when you're destroying the world when someone else is talking about how everything is pointless.

These two are so different in how they deal with things, I honestly have no idea what their relationship will be like apart from Discord messing with him, and probably getting fed up with him eventually.