The true form of the two witches Koume (fire witch) and Kotake (ice witch), Twinrova is a Gerudo queen and witch that harnesses the power of fire and ice magic. She is described as being Ganondorf's "surrogate" mother. This implies that the fatherless child was seeded from dark magic.
Twinrova is the second to the last boss in all two (or three, depending on how you look at it) games that she appears in. In all the games, you first have to fight her in her Koume and Kotake form. In this form, you deflect Kotake's attacks to Koume, and Koume's attacks back to Kotake. In the second phase, you'll have to deflect Twinrovas attacks back to her to stun her, then attack her with your sword.
She first appeared in The Legend of Zelda: The Ocarina of Time as the boss of the Spirit Temple. She also appeared as the second to final boss in The Legend of Zelda: Oracle of Seasons and/or The Legend of Zelda: Oracle of Ages. Each of the Oracle games had their own storyline that involved saving either Din or Nayru, but if you start the game with a password obtained after beating the other version, the side plot involving Kotake and Koume would eventually evolve into the capturing of Princess Zelda and the revival of Ganon for the final boss fight.
Koume and Kotake appear in Majora's Mask as good witches that make potions, but their merged form of Twinrova is absent from the game.