I...don't know if I really consider Bhunivelze much of one up until the end of this game. Before we saw his complete d*** attitude, he wasn't even a bad guy. His desire and the reason for it was somewhat understandable. Caius and Yeul, however, are largely responsible for everything that happened in XIII-2 and this game.
I'd say the ultimate villain of the XIII trilogy is Fate, every game in the saga features characters that are trying to defy their own Fates. --- FC: 2277 6869 7396 Dragon Safari: Dragonair, Fraxure, ???
I wouldn't call it irrelivent. The world wouldn't be in this state in LR if not for Caius, Noel, and Sarah's actions in XIII-2. I'd say the story of XIII-2 becomes highly relevant in that respect.
Likewise the setup that required building the artificial Cocoon in XIII-2 was a result of the actions of the heroes in XIII to bring about the fall of the original Cocoon and the creation of the crystal pillar.
The previous stories and the outcomes of those stories are relevant.
Still, if I had to pick a single character to be the main villain responsible, I'd have to pick Caius. It was primarily his actions and manipulations that brought the world to the state that it was in, and he brought that about intentionally. While Etro might be blamed for the splitting timestreams and initial leak of chaos, that was I believe unintentional. Bhunevelze, while rather uncaring for humans, is simply acting in what he views as best... despite what that does to humans. Caius's actions however were motivated by purely selfish desire and intentionally brought the world to it's knees. To me, that's what makes him the villain in all this.
You can expect Bhunivelze to be mentioned in XV also. I knew where they were going with the story in LR before it came out in Japan and in the US. It's was pretty obvious too if you know the mythology.
I forget, did Caius play a role in the state Noel's original time ended up in?
I mean, even if the end of XIII-2 didn't happen, didn't humanity go extinct in the original future? I forget what had gone wrong with this though, my memory fails me. Seems like eitherway everything had gone bad, Caius or no Caius, just in a different way.