Kuja & other vilians
Kuja and the other FF vilians have a few things in common, and a few diferences...here they will be compared! And here is a spoiler alert again...o yeah, it only has the vilians from FF6 untill 10 at the moment...I'd might be adding more from the previous FF's if I can find out more about them!
Kefka (FF6)

allright, I probably get killed for this, but I compare them anyway! What do they have in common? The first thing that comes in my mind is that they are both underrated. They are both good vilians, although they are very different. Kefka was kind of an insane person. Plus, he was pure evil, a thing Kuja wasn't. Kuja got 'insane' when he heard that he was going to die. So they have both a very different personality. Their looks are a different story. They both look 'weird' and 'girlish' (at least I thought Kefka was a girl first!). They both wear make-up (although Kefka wears a lot more then Kuja does) and they both have feathers in their hair. Maybe you'll notice them when you look good at the pic. And another thing they have in common: they are both designed by Armano! Maybe Kuja is somehow based on Kefka. (they both laugh weird! :P)
Sephiroth (FF7)
Sephiroth and Kuja are compared a lot! Some say Kuja is a rip-off from Sephiroth. But the only thing they have in common is their looks; they both have long silver/white hair. Another thing they have in common is they are both experiments. Sephiroth is born from the cells of Jenova, and as we know, Kuja is a Genome, created in a lab. But there are also a lot of differences. Sephiroth was supposed to be stronger the Cloud, while Kuja was supposed to be weaker then Zidane. Their clothing is pretty different as well, sephiroth wears a long black coat, pants etc...while Kuja has more revealing clothes.Their style of battle is also different. Sephiroth uses a long sword. Kuja, on the other hand...uses magic. Their personality also differs. Although Kuja and Sephiroth both have feelings, Sephiroth is a lot colder then Kuja is.
Ultimecia (FF8)

As far as I know, Ultimecia's motive to do what she did was quite unknown. But I've heard something about the 'Ultimecia=Rinoa'-theory. I've read some good argumentive facts about that, (one of them was of the GF Griever, wasn't the lion on Squall's ring called like that? Rinoa has it...) so I believe it's true. So I will compare out of that point of view. Rinoa once said that if her powers would get out of control, if was Ok for Squall to stop her, and I think that's the thing that got Ultimecia/Rinoa to do what she did. It proves the fact she has feelings, but she got kind of insane, wich I think was of loneliness. That's an important thing she and Kuja have in common. They both had feelings, and they went over corpses to get it away. Another thing they have in common are their powers. They both use magic spells. And theeir looks? They look both Girlish....^_^
Seymour (FF10)

Allright, now we get to Seymour. They only have a few things in common; they are both red mages and attack with magic spells. They both had control over summons (although Kuja had control over one for a little while). But there are more differences. Seymour has 'less theater'. Kuja speaks in a 'theater-like' way, while Seymour doesn't! Also he wears more 'formal' clothing, while Kuja wears more revealing ones. And allthough they both use magic spells, Seymour uses a rod as a weapon (to summon). Kuja has no weapon at all, and he used an airship 'the Invincible' to summon. Also, Seymour has natural parents, a mother and a father, Kuja doesn't. Another difference is their age; Seymour is 30 years old, while Kuja is 24.