Gaius Amidonia is the father of Julius Amidonia and Roroa Amidonia and the crown prince of the Principality of Amidonia.

Gaius is an arrogant and prideful man, who carried a longstanding grudge against the Elfrieden Kingdom, due to the war 50 years ago, where Amidonia lost half of its territory. He followed a 'military-first' policy and waited for the opportunity for revenge against Elfrieden.

(Source: How a Realist Hero Rebuilt the Kingdom Wiki)