He killed his abusive foster parents just to save his adoptive sibling and you witnessed everything what he did to them. You were his adoptive sibling.

He was sentenced and exiled to the Fortress of Meropide while you were sent to the adoption centre even though you don't want to be separated from him.

As you grow older, you visited him many times to check on him and turns out he's the administrator with the highest honorary title called "Duke" and you two promised to meet today.