Roxas is Sora's Nobody. He was created when Sora became a Heartless at Hollow Bastion. He deeply resembles Ven since he has Ven's heart.
Roxas was originally a member of Organization XIII until he left the Organization. Unlike the other members Roxas has no idea what his previous life was because of the short amount of time Sora spent as a Heartless. While in the Organization Roxas, Xion and Axel became friends but later leaves wondering why he has the Keyblade and for Xion's death.
Riku confronts Roxas and defeats him taking him to Ansem the Wise. DiZ put Roxas in a virtual world called Twilight Town and gives him a false identity and leaves him with no memories of the Organization to help not reveal him to them. While here though Roxas see dreams of Sora's past when he sleeps. Axel manages to find him and tries to stop him from merging with Sora. He fails and Roxas merges with Sora awakening him from his sleep.