I'll give it to you Chobo-Boy.

However I was hoping for the answer Garnet, the riddle refers to both the gemstone, first stanza, and the person named after, second stanza. Oh, and koi means love here - as it also means carp.

Sarah is her birth name, she fled from her birthplace for a reason, she was given the name Garnet by her adoptive parents {Branhe and her husband}, fled from Alexandria for a reason, and gets the alias Dagger. At some point, Zidane {the one referred to in the riddle} hands her the Garnet gemstone. The Garnet gemstone, certainly does teach the Bahamut summon.

The Garnet has been mistaken for the Carbuncle and the Ruby gemstones in history. And today, the Almandine {a type of Garnet gemstone} has nicknames of Oriental garnet, almandine ruby, and carbuncle.