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.