To get Raiden you need Odin. Go in Edger's castle in the World Of Ruin and go underground. When the game states that you hit something, go and check. In the jail there will be an open door. Go through it and you enter a cave. After the cave you will get into the Ancient Castle. Enter the throne room, go to the right side of the throne, walk five steps down, and press X. When you do this go into the right room, and a new room will be there. Note: If you do this you wll lose Odin and gain Raiden.