Lost Number (Final Fantasy VII)

The Lost Number is a boss in Final Fantasy VII. After entering the right combination for the safe in the Shin-Ra Mansion, this boss will attack the player party. The Lost Number consists of two halves; the purple half which performs physical attacks and the red half which casts magic. After reducing his HP below 3498, the Lost Number will change completely into one of his sides instead of half and half. The Lost Number changes depending on the attack that caused the reduction:
 * If the damage was inflicted by a magic attack, Lost Number will turn red and gain a substantial increase in its magic and magic defense stats. It will attack with powerful magic from then on. In this state, it has lower DEF, so use physical attacks.
 * If the damage was inflicted by a physical attack, Lost Number will turn purple and gain a boost in attack and defense. It will attack with strong physical attacks from then on. This form has low MDEF, so attack with spells.

If you have obtained the Choco/Mog summon spell or Aeris' Limit Break, Seal Evil, then you could use them to paralyze Lost Number, since it seems to be vulnerable to it. This will make the battle a lot easier. After its defeat, the player obtains Cosmo Memory, the Odin Materia, and a key to the Mansion's basement, which can be used to recruit Vincent Valentine to the party.