I Am Number 4 by Pitticus Lore (2010) HarperCollins

This book follows an alien teenager and his guardian as they try to stay one step ahead of their mortal enemies, the vicious Mogodorians, who destroyed their planet of Lorien.  Now, they’re tracking the nine surviving kids - Garde who are just now are coming into their legacies to Earth, and the Mogardorians intend to kill them.  But they have to kill them in order.  As they manage to do that, each of the other kids get a scar on their left ankle.  Three of them have been killed.  The narrator of the story is number four.  His Cepan, or guardian.

(James Frey and Jobie Hughes collaborate as Pitticus Lore)