You were once Morgan, prince of Ylisse. Before you had joined your mother-- joined Grima, you lead a relatively normal life as a ember of the royal family. Chrom and Robin loved you and your sister Lucina very much, and your childhood was devoid of any neglect. The two of you understood your parents were very busy and tended to leave for long periods of time, most of the time for official business. It was fine, you and your sister ended up filling most of the time with studying, training, and playing with each other. 

From a young age you had already had a grasp of magic. While your sister excelled in swordfighting-- already she had been chosen by the legendary Falchion, you stuffed your nose into books to further your skills as a mage. 

While you clung to your mothers side, your sister grew closer to your father. Your mother would tell you tales of how she met your father, and how the war was the one thing that brought them closer together. You loved listening to the stories of your mother and father, and deep down, you wished that perhaps one day you would find love like that as well. 

Except everything you once called normal had dissipated near immediately as your father was found slain in a foreign land, and your mother nowhere to be seen. 

You and the rest of the royal army searched high and wide for your mother. Much to your horror when you had found her, she was already long gone. Possessed by some demon-- no, a God named Grima had taken over her. She had claimed that she was the one to kill your father, and dared you to raise your sword against her. But you couldn't, you loved her too much to even think of doing so. 

Instead you joined her. You joined the Grimleal, the enemy of your family. With your father dead, the only parent left in your life had been Robin-- Grima, you couldn't abandon her. Your sister raised her sword against you, but you ran away. 

You've done things you were not proud of. You have killed, maimed, and tortured in the name of your mother. You have even slain former comrades for the sake of Grima.