"A new superior breed of humans, given birth by the Progenitor Virus"
This guy is quite possibly the one man we can say is indirectly responsible for every bio event in RE. He is evil at its worst, he is the Palpatine of RE, and what makes him so much more unique and frightening is his mysterious nature of constantly staying out of sight...
Once we finally did get a good look at him, we saw some type of breathing apparatus and a life support system, along with his Emperor Palpatine voice, it made him that much more sinister. It is a shame that we never got to see him at his best, although I don't know if I want to, he was so great as he is.
Most people always say that Wesker is the main bad guy of the RE series, but I think Spencer is more deserving of that title. There was a part in the Albert Wesker File that really made me appreciate Spencer even more than before, it mentions how he was in Europe, at his castle-like estate, and Wesker came in, and Spencer's response was, "Your back....", which was hardly audible over his laughing. That part in the file gave me goosebumps, it was like I was watching the scene before my eyes, and it also mentioned that if Wesker didn't know what Spencer was up to before, he would know what to make of him now.
Basically I think Spencer is a great, amazing character, 10/10 from me, and was hoping that in RE5 he would show up and put his long time plans into action, whatever they were anyway, but it was not what happened, he was killed off, via flashback, and left me wanting more.
I am not going to come up with some ridiculous type of theory for him coming back, I do hope he does, as he seemed to sure of himself while Wesker was there with him, almost as if he knew he had nothing to worry about....