Fantastic was recruited by the NCR because they required technical experts to work on the HELIOS One power plant. When asked, he claimed to know as much about solar power as anyone he knew, and also boasted a "theoretical degree" in physics. This was enough to fool the NCR recruiters, and he was given the job. If told that he doesn't know what he's doing, he disagrees and insists that he knows exactly what he is doing, he just doesn't know what effect it has. When asked to elaborate on his working environment, his explanation starts promisingly enough - "That console there controls the main power grid." - but soon degenerates, ending with "That slot over there makes sparks when you put stuff in it."
If the player chooses to kill the NCR in That Lucky Old Sun and leave Fantastic alive, upon returning they will find that Fantastic will be wearing Legion armor and will have killed Ignacio Rivas. If they then talk to him he will say "Hey man, when in Rome".
Alternatively, if the player chooses to side with the NCR, Fantastic can be found at Hoover Dam. He gets a promotion to the position of "God Damn Dam God" in his own words.
When first meeting Fantastic the player can state he's here to replace him. Fantastic gets worried and tells the player that he needs this job for his chem addiction, and with a Speech check of 25 the player can get 100 caps from Fantastic for not taking his job.
If killed after completing That Lucky Old Sun, an NCR guard will remark "Maybe that guy Fantastic wasn't an idiot, maybe he was a genius," when exiting the facility. Note: This does not make the NCR hostile towards you, nor does it affect NCR reputation.
"Fuck, man. I push buttons, I turn dials, I read numbers. Sometimes I make up little stories in my head about what the numbers mean."
"They asked me how well I understood theoretical physics. I told them I had a theoretical degree in physics. They said welcome aboard."
"No, man. I know exactly what I'm doing, I just don't know what effect it's going to have. That console over there controls the main power grid, that button there makes a crazy noise, and if you put stuff in that slot sparks come out..."
"But the mirrors outside aren't aimed, so right now we're running at one percent efficiency. And I guess that just isn't good enough for some assholes."