Spy is a Unit ability which allows you to draw one of two cards from your deck. It is a signature ability of Disloyal Units .
In The Witcher 3: Wild Hunt, the ability allowed you to draw two cards instead of one, and the Units generally had lower Strength values.
When Played: Draw 2 cards from your Deck, one face-up and one face-down.
Pick one card and put it in your Hand. Return the other to your Deck.