Anyone have a cat that is only aggressive to one person?

We have a 6 mth old neutered male cat. We discovered yesterday that he had sprayed by our front door. After searching the entire house with a black-light, we realized that he only sprayed by the front door. We cleaned it, used Feliway, and so far no more problems with the spraying. Stranger still, when our friend came by last night, the cat growled at him when he saw him through the glass door, and last Saturday we had our friend over for dinner, and the cat kept sort of attacking the guy's feet. This cat is completely friendly to everyone else that he comes in contact with, including our other 5 cats. Why the aggression towards one person? Anyone else have a "watch-cat"?