I forgot a old charater: GLaDOS, which is the Combine Dispatcher (the voice you've been hearing during your half-life 2 journey), she is your final boss.
I know of enemies. The Mortar Synth, Crab Synth, Robot Spider with a Soldier inside called the Webly, Zombine Torso, Wasteland Scanner, Defender Scanner (a regular city scanner that shoots laser like a strider), Combine Super Soldier (a synth soldier seen in beta), Combine Assassin, and Combine...