Jul needs to stay. He's a very good "bad guy". His motives and character have been fleshed out extremely well. It shows that Sangheili aren't just dumb bipedals going "Human bad. We kill", but an interesting, multi-faceted people. He makes us sympathise with Sanghelios and see them not as faceless killing machines, but a people struggling to survive. I will be extremely disappointed if he's killed off in Spartan Ops.