Julius Amidonia is shown with light, short hair in the manga and with brown hair in the light novel.
Julius is a vain, egotistical individual, and has dreams of grandeur and conquest. However, his ability to govern a country is severely lacking as he does not understand the needs of the Amidonian citizens, nor is he able to solve domestic issues, since like his father, he follows a 'military first' outlook.
During the rebellion of Georg Carmine and Castor Vargas, Gaius and Julius launch an invasion of the Elfrieden Kingdom in order to reclaim the territory Amidonia had lost a half-century ago. The goal was to take back the farm-able lands or the 'bread-basket' areas. This was crucial to Amidonia since the country was mostly mountainous and had little in terms of food production. With nothing but mining as revenue, the country was constantly suffering food shortages.
The aim was to annex those valuable lands with no fear of reprisal from the Grand Chaos Empire. Since Elfrieden had never signed with the Humanity Declaration Treaty, and therefore would not be able to use the first clause of no border changes due to military action, both father and son felt they could take back their country's former territories with impunity.
However, both were outwitted by Souma Kazuya's deception tactics, and the Amidonian Army soon found itself being pushed back with considerable casualties, and forced to retreat back to its country, once their capital city of Vannes was captured. During the Five Day War, Gaius Amidonia met his end in a suicidal charge, in an attempt to kill Souma. By the time Julius arrived at Vannes, it was already fully occupied by Elfrieden's forces and the Amidonian Army had neither the strength, numbers or resources to retake the city.
With Julius now the ruler of Amidonia, (unofficially, since a coronation ceremony was never held after his father's death), he was forced to appeal to the Grand Chaos Empire in order to envoke the first clause of the Humanity Declaration Treaty, in order to get Vannes back. After a lengthy negotiation, he was able to regain the capital city, but much to his consternation, was forced to pay enormous reparations for invading Elfrieden in the first place.
Julius' troubles did not end after the occupying forces left Vannes. Due to the citizens enjoying the entertainment, food supplies and overall freedoms granted during the occupation, they were very resentful of their ruler, once Julius began restricting them in an attempt to return things to the former status quo. It wasn't long before rebellions and riots began appearing throughout the country, In an attempt to put down a major rebellion in the north, Julius led his army to combat the threat. However, that allowed the people to revolt in Vannes, declaring allegiance to Elfrieden and giving Souma a reason to occupy the city again.
With no way of fighting both the rebellions and the re-occupation of Vannes, Julius was forced into a self-imposed exile, while the whole country of Amidonia was eventually annexed by Elfrieden.