Main Plot And Final BossEssentially every human and Fal'Cie in this game is considered an "Orphan" of the Maker. Or at least based off of what we know of the entire Fal'Cie point of view. The Maker is the creator of Humans and Fal'Cie and by leaving them, he's essentially "Orphaned" the races to survive on their own, which makes Orphan a perfect choice for the Final Boss. Especially since the Fal'Cie are so willing to be rejoined with their creator that they're willing to attempt to use their Brethren as sacrifices.