Who is Constantius Augustus from Julian and what is their importance?

Constantius Augustus was Julian's cousin, as well as the man responsible for the murders of both his father and brother. The one thing that saved Julian from death was his position as Constantius' only living relative. He had no children and believed there was no prospect of children in his future. Believing himself unable to father a child, Faustia's pregnancy would have been a shock. Constantius, however, would not live to see the birth of his only child..... a daughter.