Necron......Filler or Final Boss?

[The Crystal]

necron exploding and imploding and just dissapearing, is quite misleading but i believe necron only actually appeared when kuja destroyed the crystal.

necron wasn't just sitting there waiting for everyone to lose the will to live. he appeared when he was needed and dissapeared when he was no longer needed and he may yet return when it is time for him to reduce everything to the zero world.

The problem with your theory, is that Kuja didn't destroy the Crystal. He used Ultima against your party, not against the Crystal.
And like i said, if Necron was just retreating, he would just dissapear or go away. He would not explode/implode without any reason. The only evidence that you have, that Necron didn't die, is a quote from him. A quote that is very like what Zeromus said, but we know that Zeromus is already dead, so, why Necron cannot be dead too?
We have just a quote from him, saying that he is immortal. And we have a little video/cutscene showing his dead. What is more valid as an evidence?

And tell me something, why does necron have to be dead just cuz zeromus is? If you hadnt noticed, Necron is not Zeromus. Quit acting like he is. And how do you know he didnt die? He imploded, his physical body collapsed in on itself. Just because his body went away does not mean he was killed. He's "the darkness of eternity", he's an entity, and most likely does not need a physical body to survive.

And tell me something. If Necron wanted to kill the party, and he is THAT powerful, why he didn't use all this power to kill them? And if he just retreated in the end, why he choosed to blow up himself, instead of just go away?
Even if he didn't die, he was defeated. A being that can destroy the entire universe, was deffeated by a group of mortalls?!

And Garland said that what we see is only the physical part of the Iifa Tree, meaning that exist a spiritual part of it too. After Kuja kill the party in Crystal World they go to the spiritual world and find Necron there. And if you see Necron's face, you will note that his face is like the faces in the wall of that place where you fight Ark. And these faces, were created by Terrans.