I just read all the articles again, after winning both games again, and i came to a little conclusion:
Nu may work for the entity, but who is the entity? i think it might be another creature who lives symbotically with the core of the earth, if this is correct lavos may have been trying to kill the entity, so Nu start to dissapper without a way to be powered. also it would make sense if he kept on near dying he would need CHAMPIONS to take out lavos for it. when the guys from chrono trigger weren't enough, he needed Stronger champions and might have had a hand with porre's uprising in chrono cross so he could get new champions, and might of manipulated the creation of the time devourer to Make sure they could kill Lavos for good. but the as far as I know rumored Chrono Break shows that either theres another lavos or once again he needs champions maybe to fight the orginal Lavos even! maybe after they fail the entity will grap all his champions for one final attack with him in it. that'll be cool! but probaly hopping for an impossiblity
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without Nu to keep track of things, the world became more and more mechanlized. then you may ask, "then why do a few still exist?" the one in the blue pyramind might have been powered by the seal Queen Zeal ordered there, that's why he dissapers, then the 1 in 2300 AD, didn't we learn nu's where organic creatures when we fought them in 65,000,000 BC? or did it take to when we scratched a nu's back? the fact it had a switch tells me this is a cyborg nu, with it's own power core. Last spekko, well he's a mage of awesome porportions, but he said that Magus could teach him a thing or 2, and since it's **generally** accepted that Magus used magic to create a disgue, couldn't spekkio do it to?
And then Time travel comes into play, remeber that Nu we shoved off in the ocean Palace? could he be sent by the Entity back in time to save him? and since the future is now ok, but the entity is dieing there are no Nu there. And maybe the entity keeps on having NEAR-death fights with Lavos, creating Chrono Cross, and maybe even the next game. Oh and I keep getting my gurus mixed up. because the way I see it the guru of time should be at the end of time, BUT only the guru of time should be able to make a time machine. So which one's the guru of REASON? for he probaly either wrote or had the book that talks about Nu.
but those are my thoughts, i'm 13 and had alot time to think I also watched star trek all day, all the time travel eposides and 1st contact movie, so time travel me trying to understand but TO CONFUSING.