The Ur-Didact (as it was technically the Bornstellar-Didact that Bungie created) is a very interesting character with a lot of depth when you look at his backstory. That's the thing really, if you play Halo 4 without any knowledge about the Didact then he comes off as a bit of a shoddy villain, but if you look more deeply at his character in Cryptum, Halo 4's Terminals and the upcoming Silentium then he's one of the most fascinating, well-rounded characters in the series. Jul 'Mdama is another great character, despite the disagreements I have with much of what Karen has done in the Kilo-5 Trilogy, I think that she did a tremendous job developing Jul in The Thursday War and 343i continued that in Spartan Ops. What I find most interesting about him is that his goal is [i]not[/i] the eradication of humanity like the other Covenant villains, at no point does he ever say that that's his goal. Jul is clever and manipulative, using the resources at his disposal to acquire knowledge and ultimately restore the Sangheili to their former, pre-Covenant glory rather than be stamped out by humanity once they recover. 343 Guilty Spark was treated like a complete joke by Bungie from Halo CE-3, but in CEA's Terminals Kevin Grace really developed him well as a tragic and lonely person. Greg Bear further developed this in Primordium where it's revealed that Spark is a fragment of Chakas, who was also given a great deal of backstory. Phillips and Black Box are more examples of characters that Karen wrote well, I really enjoyed reading the bits in The Thursday War where Phillips was having to fend for himself on Sanghelios and his witty commentary on things. Faber is awesome, one of the more interesting antagonists in the series - an almost Palpatine kind of figure who is manipulative of others in politics, but brutal and uncompromising in practice. Sanctioning the creation of the Halos to dominate life, using it to crush the San'Shyuum rebellion and sterilise them, performing tests on the Flood by letting them infect humans, and marooning the Ur-Didact in a Flood-infested system. The Sangheili Shipmaster in Halo: Evolutions - The Return (written by Kevin Grace, the same guy who single-handedly made 343 Guilty Spark a decent character) was really well-characterised in the short time he was present. Likewise with Soren-066, the deformed Spartan-II who goes missing on Reach in the story Pariah. Overall, I think that building characters is one of the main strengths of the writers at 343. Even those who are only present for a short time (like Doctor Tillson, as the OP pointed out) are really well characterised and can often make you feel sympathy for them.