Well.. Roxas took Riku down the first time.. Didn't you see that part where Riku turned to Ansem to defeat Roxas.. Riku won that battle.And if Riku didn't defeat Roxas.. Sora would have troubles waking up. Ansem the wise told Riku to defeat Roxas and to bring roxas to him. So he did. Thats why he couldn't remember Axel because Ansem earsed roxas memories so it would help Sora..
So i make myself!
And plus if it wasn't for Sora, Roxas is nothing.. no keyblade.. no skills..