"Hey Brother... Do you still believe in one another?" 

 Diego is the survivor of an old civilisation from the most remote territories of the realm of Metamoor isolated from the capital of Meridian know as the "Shifters"( due to their abilities to take any form by absorbing the vital energy of a chosen host ).  

 In the middle of the revolution against the tyrannic rule of prince Phobos both Diego and Caleb are taken under the wing of the former rebel leader Julian, and grow up together forming a strong brotherly bond. Eventually as Caleb takes leadership of the rebellion Diego becomes his most trusted comrade and right hand man. 

 However Diego's growing bitterness over the disdain he endured from the ones around him ( since male magic users like himself are shunned for being a big taboo amongst the Meridianites ) leads him to betray Caleb and steal his identity out of spite and jealousy. 

Caleb has his life stolen from him and is incarcerated for years while Diego replaces him at the side of Queen Elyon, the rightful Escanor heir.