Agent Boon is a character that first appears during the quest: Secret plans?!, where you build him a jetpack. The quest has 4 parts. After you finish the quest, you will often see him flying over your town like other visitors in his jetpack. If stopped, he will usually spend around two or three hundred coins, 1-3 times in the same shop (so 200 to 900 total). You won't see him walk up to any shops, but you should see money being spent at the store he stops in front of. He will spend much more during a Fiesta.
"Hi. You. I am Agent Boon. Of the Secret Agency of Secret Agents."
You will only see Agent Boon hovering over your street after you finish his quest.