I like it, but who they spying on?
Everyone!
Nah, jokes aside, I figure the character would be useful when the party crew need to have a little bit of tact when trying to procure something (like say a ship full of gold) or when some clever reconnaissance work needs to be undertaken. The way I see it, when you're doing potentially bad things (or even good things), sometimes you need someone who can fight and other times you need someone who can accomplish a task with the utmost discretion.
Another valuable trait of said character would be connections to people who deal in information. Say for example, a most beloved crew-member is captured, well how does the party crew find them? Perhaps, the mysterious spy can ask some old colleagues to ask the right people the right questions?
Who hired them?
Nominally, the spy would work for the Captain...although who knows? Can you ever trust a spy? :D