Icarus. Everything about Icarus. From his insanely scary voice to him just about stalking you all through Paris, it's one gallon of nightmare fuel after another.
He first appears when you're down in a sewer. MJ12 soldiers and their robots are right above you, and they're looking for your head. You pass on, and suddenly, your infolink rings, revealing a demonic voice and a creepy looking eyeball. "I now have access to your systems."
Answer a ringing phone in an empty office building, and the voice on the other end (Icarus) will tell you that he is right behind you, and will soon be "a part of everything in your world." Check it out here.
Take the time to look at a Paris public terminal and you'll find that one of the entries is titled "ICARUS FOUND YOU". Take a look inside, and it's just those three words, over and over, in a > shape.
RUN WHILE YOU CAN
Also, freaking greasels. They're weird little chupacabra-looking creatures that spit acid. They're tiny compared to you, so you never know if you'll run into one in a cramped ventilation shaft. Also, they make this really creepy noise whenever you're near them. To top it all off, they're very hard to kill.