Walter Phebus is a former brahmin rancher that has had his land taken by force by Heck Gunderson and his men at gunpoint. Phebus, deciding that Gunderson needs to be taken down, has followed him to New Vegas seeking his revenge.
Walter and his wife will continue to mill around the Strip even after they choose to return to their ranch.
It's possible to pickpocket Walter Phebus infinitely. Each time you attempt to pickpocket him, you lose karma but the item is never removed from his inventory. If you have a high enough sneak skill, you can exploit this glitch and get the Artful Pocketer achievement very easily. The item you select is never actually added to your inventory, however each time you take the Karma loss then exit the inventory menu, it counts as a successful pickpocket. (confirmed on multiple characters)