I keep hoping Daniel turns out to be alive I mean he was a supposedly an every day web designer and I just can't imagine what division who had the "Shadow Walker" would need from him. So I'm hoping he turns out to be a villain he doesn't even have to be old to the spy game.
Percy may have had numerous people stashed some where like that guy who did experiments on peoples brains turning them into sleepers. Now your probably wondering how a normal guy could turn into a diabolical criminal master mind?
I suspect if Daniel was held somewhere he may have been subjected to the weird mind programming and the regiment that the Guardians use they had to of tested it on someone. All that alone could make someone snap and go insane it's also not a stretch to think that Nikita's not really the first to get out of Division. I mean Daniel could have been former division and could have been the first real escapee and he may have been so mentally unstable that he was viewed as a disgrace to the program and Percy wiped any trace of his time at division.
Though wether it was because of something Division did or because he really had mental problems though I doubt Division would keep someone like that around for long and even if Nikita did calm his inner psychopath she probably would have picked up on him being unbalanced.
Ideally I think this angle might have a lot of potential because this type of enemy could have the same leadership skills and be smart, crazy and unpredicatable as the Joker(The Dark Knight) with the strength and skill of Bane(The Dark Knigh Rises) and this kind of foe is probably the ultimate enemy for Nikita especially if he's genuienly in love with her and his love is the crazy stalker type love as in the "I did all this for you Nikita why can't you appreciate that?" type of insanity.