Who is Julian the Apostate, later Julian Ceasar and then Julian Aug from Julian and what is their importance?

Julian was a small child when he was literally ripped from his home as his father was arrested. There's little doubt that this early encounter had an incredible impact on the person the man would become. His mother died at an early age, and he never saw his father again. Julian had one half-brother, and the two were never close. Soon after his brother became "Caesar," he was killed by the same man who killed Julian's father - Constantius Augustus, Julian's cousin. Julian is then named Caesar as Constantius has no other living male relatives.