Yasuo works at a ramen restaurant and is dating the pretty news reporter Yukiko. However, things aren't going well for him financially and after a customer leaves a green jacket behind, Yasuo starts adding to his income by walking the crowded streets of the Tokyo metroplex and stealing people's wallets. With this, he's able to buy his girlfriend a nice ring even though she says he didn't need to buy this for her.
Over time, Yasuo becomes obsessed with becoming Lupin to the point that he reads and watches everything regarding him, he even works with Jigen for a time.

However, when Yasuo learns of the Ice Cube diamond, he knows that if he can steal this, he will be the real Lupin.

(Source: Lupin Wiki, edited)