Welcome to the Tower of Zot!"
Cindy is a boss in Final Fantasy IV. Cindy, Sandy, and Mindy are called the Magus Sisters. They are servants of the wind boss Barbariccia. They attack Cecil and the party on the fifth floor in the Tower of Zot just after a room with a save point.
The Magus Sisters attack using their Delta Attack, in which Sandy casts Reflect on Cindy, and Mindy uses a level two spell or Bio on Cindy, reflecting it back on to Cecil's party. Cindy also has the power to revive her allies if they die, so it is recommended that she be the first target of the party.
Another strategy is kill Sandy first, while Cecil or Tellah heal the party. After defeating Sandy, Tellah should use Dispel on Cindy, then use an -"aga" spell on Cindy. Cecil should cover Tellah. Cindy should be defeated before she uses Reraise. After defeating Cindy, Mindy will cast Bio and a -"ra" spell on herself.
Due to her nature of reviving both Sandy and Mindy after her two long consecutive turns, it is possible for a player to rack an enormous amount of experience point and Gil from this single battle, if prepared thoroughly. When Sandy or Mindy (or both) is killed, and then resurrected by Cindy, the amount of both experience point and gil for the resurrected sisters will be accumulated throughout the battle. Thus a player can create a loop of killing Sandy and Mindy, and then letting Cindy resurrect them--to accumulate the experience point and gil. It would also be wise to heal Cindy as well if she got damaged to prevent her from dying.