Necron?

[Goldenboko]

But anything that they said about Necron they didn't spell Necron's name anytime, this is obvious...

Foreshadowing. Its means to give hints to future events. To me it would be odd if ANY of the characters knew Necron's name because how would they know, Necron has never been seen by any living human eyes at the time. What matters is theres many points where they tend to hint toward Necron...

The problem now comes whether you believe Squall of SeeD or the idea that Necron is the antitheses of the Crystal.

It was a good answer...But how they saw Necron even it has never been seen by any living human eyes?? Is Kuja's last magic, Ultima really killed them all? Sorry but please do not give any spoilers because I didn't beat the game, I stuck with Necron, the last boss...

There's two different main theories.

Squall of SeeD's states Necron is the main core of the Iifa Tree. I'm most convinced by this one.
If you notice Garland states The only way to live in a world without fear is to live in a world where everything is dead. That is shockingly similar to what Necron states he must send everything to the Zero world (a world where there is no life). This seems to say their intentions we're the same making it highly possible Garland DID create Necron.
Now Garland states the main function of the core unit of the Iifa tree is to disrupt the Cycle of Souls occurring in Gaia. He says when you die, your soul returns to the land so life may continue, and the main unit is stopping this.
This would explain why when the characters are defeated they see Necron. Because Kuja really did kill them, and now they are up against the unit disrupting the cycle of souls. What makes this more convincing is the fact the location they are sent to (the Hill of despair) has the moaning of souls in the background.

The other theory is far less convincing to me. It states that Necron is either the Opposite of the Crystal and appears b/c the Crystal is destroyed -or- that Necron is the powerful figure that was sealed in the Crystal by the ancient Summoners, and escaped because this Crystal was destroyed.

I don't believe this theory because Necron says "I exist for one purpose... To return everything back to
the zero world, where there is no life and no crystal to give life. In
a world of nothing, fear does not exist. This is the world that all
life desires." Necron says "I need to destory the Crystal to stop life", which clearly contradicts this second theory.