When Cell was premature and growing in that capsule, he was mentally programmed by Gero; his voice recording was played in Cell's mind over and over again. In that recording, he goes something like, 'Cell, you must listen carefully. You are my ultimate achievement...' Dr. Gero created Cell's mind and body from scratch, and so he's able to have greater control over his mind than the other androids. Gero's words are so deeply ingrained into his mind that I'm sure he Cell would
at least hesitate to do harm to him. I'm also sure that Cell has great respect for him; after all, if it weren't for his creator, he wouldn't be
'The Ultimate Perfect Entity'.
Damn, Cell's so vain.
Now, obviously, Cell has been programmed to have these characteristics (including his unnatural superiority complex). And if Gero can make him have these, he can program him to be loyal, too. Hehe, too bad he couldn't do that with #17 and #18.
EDIT: Wait. When Cell absorbs the androids, he takes on their characteristics- mind and body. So, with #18's and #17's minds being a part of Cell, wouldn't he be less 'pure', from, say, when he was first born?

So maybe, he wouldn't be
that loyal to Gero after all. And also, I'm finding a flaw with my above original post. Cell only really became 'Vain' after he absorbed both androids (that's evidence for Cell taking on their mental characteristics- #17 especially), and because of this inherited characteristic, he's got to have inherited #17's defiance, too. Before that, he was like a wild beast, lurking in the shadows and suckin' people up. Not much personality there.
