The Avatar
Here is the character who doubles as both one of the protagonists and as one of the antagonists, the Avatar (also known as Robin and Grima).
Info:
[collapse=About Robin *SPOILER*]He/She is the child of Validar, leader of the Grimleal. Bred to help bring about the the return of the Fell Dragon Grima. In one timeline, this happens, where after playing a role in Chrom's death, Robin becomes Grima, who then goes on to terrorize the world. In the timeline you play as, this is averted thanks to planning on Robin's part as well as the interference of Chrom's daughter Lucina, among other things.
First Appearance: Fire Emblem: Awakening
Wiki: http://fireemblem.wikia.com/wiki/Avatar (Awakening)[/collapse]
[collapse=About Grima *SPOILER*]The Fell Dragon who terrorized Ylisse centuries before Awakening, this dragon is descended from the Earth Dragons. Grima was stopped by the First Exalt and Naga, being sealed away. Grima returns through Robin, who goes on terrorize the world. Lucina goes back in time to change the future, Grima also does so. Grima tries to change Robin, but the attempt makes Robin amnesiac. Several events later, Grima!Robin is fought and beaten, being either killed or sealed away.
First Appearance: Fire Emblem: Awakening
Wiki: http://fireemblem.wikia.com/wiki/Grima[/collapse]
CHARACTER DISCUSSION
So what do any of you say about the Avatar. Be it movesets, costumes, and anything else that involves the Avatar, Robin, and Grima.
