I Am Number Four
The antagonists in this book are the Mogadorians which are an alien race. They have strength, hatred, and determination. They started out by nearly destroying the Lorien race (another alien group). The reason these aliens started to attack the Loriens years ago is still not given in the book. However, now they are going against them to completely destroy all Loriens. If the remaining Garde (aliens with special powers) get their powers before the mogadorians catch them they will be able to fight back against them. The Garde members don't completely have their powers yet so they have to stay hidden. The Mogadorians want them dead (for no specific reason why) so they continue to hunt them on Earth. The catch is that they may only hunt the nine Garde members in order of their numbers starting with one and going to nine. John is number four and is the one who is hunted now. He is the protagonist at this point and must stay out of sight from the Mogadorians or they will simply kill him on the spot to continue to eliminate the Lorien race.