G'Kar famously described Babylon 5 by saying that "No one here is exactly what he appears", and there is perhaps not another character that epitomizes that as much as Vir Cotto, the aide to Londo Mollari, who appears in the first episode as a bumbling, inept man who was only given the job so that his family could get him out of the way, and is developed throughout the series until the last episode shows him as the Emperor of the Centauri Republic.
Of course, real character development doesn't mean just having a character with one image and then revealing that image as being totally false. Vir Cotto is shy, unsure of himself, and (incidentally), a virgin, and his character development doesn't happen immediately. He first is shown as being a reluctant accomplice to Londo Mollari's war making plans, with qualms about what is going on, but without the courage to speak up. When he apologizes to G'Kar for the Centauri aggression, G'Kar responds simply that since Vir can not apologize to the dead, G'Kar can not accept his apology. So from this impotent frustration, Vir starts taking action, showing bravery by smuggling Narn out of prison and labor camps. And then later on, he progresses to taking even more firm action, being the one who finally drives in the poison dagger that kills the mad Emperor Cartagia.
And this last point is again where we see the multiple layers of character development. Even when he had the courage to murder the insane, genocidal Cartagia, Vir is still the gentle soul he has always been: he is reduced to tears at the thought of causing harm, even to a madman. Throughout the arc that shows Vir's growth in moral and personal courage, he is still possessed of his original characteristics and mannerisms, which are both indicative of his character, and are entertaining to watch.
A mention of Vir Cotto can't be made without making a mention of Stephen Furst, the talented actor who plays him. One of the most skilled parts of Furst's portryal is how the body language and appearance of Vir gradually change to reflect his maturation. In the beginning of the series, he is hunched over and stammers, both of which gradually (but not totally) disappear as he becomes more self-confident. At one point in the series, he also loses a great deal of weight, and although this was due to health concerns of the actor, it also makes Vir Cotto appear more imposing.
Vir Cotto is one of my favorite characters on Babylon 5, and shows a multifaceted development that is rare in a television show.