From the short amount I've read, you broadcasted the fact that there would be little or no consequence for his actions. Not wanting to be seen,i.e. liking the invisibility factor should always be outweighed by personal preservation. Fight, not flight (or in this case, reporting the little freak to the owner) is the appropriate response.
To answer why some kids get picked on while others don't? The targets for bullies usually back down, or attempt flailingly to fight; they get really mad when picked on. I fixed this in school by becoming something of a comic; if the bully pushed me from behind, I'd flow with the direction, tumble forward and roll into a standing position again, then thank them for helping me demonstrate my agility. I would laugh at the insults they threw at me. Eventually they picked someone else who didn't have "fun" being picked on. My brother took bullies out because he was head and shoulders above them all, and he's my younger brother. We both dealt with it the only way we could, I with my humor, him with his power and size. Everyone has the strength to defend themselves in some way, you just need to look for yours, Gilda. Let them know you aren't a target, and you won't put up with anyone harrassing you in any way.
P.S. I know I'm small, but even as small as I am, if someone had decided to grab my balls or somethin', they would have gotten hurt. I have a sense of humor and everything, but that is my knee-jerk reaction when people invade my personal space without asking or having permission, dammit. 