Kingdom Hearts II
When investigating the Seven Wonders of Twilight Town, Roxas checks up on "The Doppelgänger" wonder, and ends up in front of a waterfall. He sees a reflection of himself, which turns into a dark Roxas clone and engages in a battle with him. Shadow Roxas is last seen in Kingdom Hearts II after Roxas and his friends leave the waterfall, peering out ominously until dissipating into nothingness.
Shadow Roxas is a very easy boss to defeat. He uses a Struggle Bat against Roxas. The simplest method is to Guard or deflect his attacks, then hack at him. However, he does not give any experience once he is defeated.
Another strategy is to just hack at it. If you attack right after you knock him away with a combo, you will succesfully block it's attack and send it staggering.