G'Kar is a wonderful character. Full of good and bad, you can despise and admire him (sometimes in the same episode). He is a sort of spiritual leader among his people and, like all Narn, he hates the Centauri. After a Vorlon-induced revelation he became more philosophical, more serene and perhaps more determined. His character has developed into someone who is cool and calculating. If you were to meet him in a pub you might notice him playing a flute fashioned out of a Centauri shinbone.

He has had a tough upbringing. After watching his father die hanging from a tree for three days for spilling some wine of one of the Centauri masters, G'Kar will not rest until the Centauri were wiped out.

The Centauri have occupied Narn twice and their occupation can, at best, be described as brutal. As Ambassador on Babylon 5, G'Kar was regularly plotting against the Centauri and headed the Narn Underground. He was also admitted into the Army of Light and has provided much needed knowledge and resources in the fight against the Shadows.

The most electrifying moments in the series come when G'Kar and Londo are in the same scene. There is always a battle of wills, a confrontation, and yet neither ends up as the victor. Both Londo and G'Kar are vicious, ruthless, humorous and passionate about their people.

The Narn have a very thick skin as was evident when the Centauri were torturing him.
He is also bloody ugly.

G'Kar had a very nasty run-in with the Emperor Cartagia of Centauri Prime in Series Four.  He was captured and tortured (basically for amusement) but finally got his own back with the help of his old enemy Londo Mollari.  For helping Londo assassinate Cartagia, G'Kar was able to free his homeworld of Centauri and both he and Londo now seem to have a strenuous and fragile appreciation of each other, not a friendship exactly, but it's getting there.  

Now, whenever Londo returns home on Diplomatic Business, he takes G'Kar - As his bodyguard, but also I think, because it makes a rather striking diplomatic point!