Who was Kilroy, the guy who kept writing "Kilroy was here" everywhere he went?

From what Ive read. It was the advanced unit during WWII who declared towns free from germans

It was WW2, the American army marking their progress.
At times, the army moves in pieces & a follower squad may be in confusion whether they are in a enemy dominated territory or whether they are safe. This marking was probably to give them a sign.