Gatsby Colbert held the position of Finance Minister for the Principality of Amidonia before its annexation. He exhibits a timid and thin physical presence with short hair, mirroring his reserved personality. Despite his shy and easily intimidated disposition, he displays considerable common sense and economic expertise. During Amidonia’s war councils, he vehemently opposes invading the Elfrieden Kingdom, arguing that insufficient resources and finances would lead to defeat and devastate Amidonia's failing economy. He further warns this action breaches the First Clause of the Humanity Declaration Treaty. His cautious position prompts King Gaius Amidonia to denounce him as cowardly, resulting in his dismissal and house arrest.

After his removal, Colbert flees into hiding alongside Princess Roroa Amidonia. When the One Week War ends, he partners with Herman Neumann to avert an invasion by the Republic of Turgis. Upon Amidonia’s annexation and reorganization into the Kingdom of Friedonia, Kazuya Souma appoints Colbert as Finance Minister for the new realm, acknowledging his economic proficiency.

Colbert’s core ability lies in economic management, specializing in eradicating wasteful expenditure and securing funds. His skills earn high esteem from diverse figures: Julius Amidonia seeks his service upon becoming Crown Prince, Princess Roroa—herself a capable economist—trusts him as a subordinate, and Souma immediately tasks him with overseeing finances for both Elfrieden and Amidonia post-annexation. Though inherently lacking confidence, Colbert exhibits courage by challenging intimidating authorities like Gaius and Georg when advocating for his people’s welfare. The formidable warrior Georg even commends him as a promising young man after their first encounter.