Immune to Status
Return, Confusion, Silence, Transform, Berserk, Stone, Slowly Stone, Manipulate, Death
In addition to the item officially won from this battle, you will also find Odin and a Basement Key after your victory.
Lost Number is vulnerable to all types of assault at first, but when one side dies, then the other takes over and the rules change. If his purple side dies, the red side stays and attacks you with only magic, and is very resistant to magic. If the red side dies and leaves the purple side, it attacks with only physical attacks, and is very resistant to your weapons. Fight the red side with physical strikes, and the purple side with magic. He's vulnerable to Stop, too, so you can use Seal Evil and Choco/Mog to paralyze him. A wise thing to do is break Aeris' Limit before the fight, and use Seal Evil on him right away.