Eiko Carol, is one of the playable characters that joins the party in the second disc of the game. She is the only person who appears in the game with a Summoner's Horn. While Eiko and Garnet both share White Magic and Summoner abilities, Eiko uses primarily white magic with her summoner powers taking a secondary role, indicated by the order of said skills on the battle menu. Her summon abilities are usually holy-based summons or summons that add supporting effects. Some examples are Phoenix, which revives all unconscious party members, and Madeen, which causes Holy damage.
Eiko is one of the two last surviving Summoners of Madain Sari (the other being Garnet). Eiko's family was not in Madain Sari when it was destroyed by Garland. They returned to the ruins of the village four years later, when Eiko was very young. Eiko's parents died early, leaving her grandfather to raise her. However, he died as well when she was five. Fortunately for Eiko, the moogles in the area decided to take care of her. She became very mature for her age and helped them as well, stealing food from the nearby village of Conde Petie for them when it was needed. Zidane and the rest of the party found her caught in a tree after she fled the scene of one of her thefts. After they got her down, she decided to show them Madain Sari and then to join the party, despite the fact that her grandfather had told her not to leave until she was sixteen. Eiko develops a huge crush on Zidane, and considers Garnet to be her rival for his affections, though gives up her pursuit of him when it becomes clear to her that Zidane's true affections lie mainly with Garnet, not with her.
Eiko has a guardian female moogle who goes by the name of Mog. During an extraction of Eiko's eidolons (the same process Thorn and Zorn used on Garnet/Dagger in Alexandria) Mog goes into a trance to protect Eiko and transforms into Madeen. After the battle, she gives Eiko the ribbon she wore so that Eiko can summon her at will.
After defeating Kuja, one of the scenes in the game shows Eiko referring to Cid and Hilda as her parents. Although the Regent had never established an adoption, his wife, Hilda, acknowledges it.