Eiko Carol (エーコ・キャルオル Ēko Kyaruoru) 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.
Source: Wikipedia, "Garnet Til Alexandros XVII", available under the CC-BY-SA License.