Riku is a character from the Kingdom Hearts series. He wields the Soul Stealer Keyblade and fights against the Heartless, which he previously had control over. Outside of the Kingdom Hearts series, he makes appearences in other Nintendo games. Recently, he was killed for good by the Pied Piper. However, whether this was canon or not is debatable.
In Mario Guitar World, he is a unlockable character at the store. Buying him also unlocks the song "My Curse" by Kill Switch Engage.
Riku makes his playable debut in Super Smash Bros. Evolution. His regular and special attacks are performed with his Keyblade. His Final Smash is the Dark Aura attack. He is best friends with Lucas (Mother) and Daisy on Destiny Islands.
Riku is on Luigi's faction in Nintendo Civil War. He wields his "Ansem Riku" abilities in this time. He betrays the faction and kills Dimentio to throw all the worlds into everlasting darkness. He is beaten by Mario's faction and is knocked into a coma. Dimentio was then resurrected by the Neverworlder.
After being knocked into a coma, he was killed by the Pied Piper (the Piper was the result of the reborn Dimentio's fusion with Boo and Luigi). The Piper then absorbs his Ansem Riku powers and uses them to ensure that Riku will never return from the dead.