Diago is a middle aged man with caramel colored skin and large, devilish eyes. He lacks the traditional horns and tail of his kind. His hair is grey and kept in a tight, waist-length braided ponytail that has never been cut. He sports a sharp, manicured goatee. His hands appear permanently charred, which he keeps hidden beneath elegant silk or leather gloves. He wears high quality battle scarred attire that reflects his status as a majordomo and soldier.

Diago is a study in controlled intensity. He is consistently calm and charming, possessing a sophisticated manner that masks a cold, pragmatic efficiency. He believes that order is maintained through both silver tongues and sharp steel. While he prefers diplomacy and the quiet collection of secrets, he does not hesitate to use psychological or physical pressure to protect the interests of the Martinez family.

Once a devoted priest of the Spanish orthodox faith, Diago's keen eyes for investigation led him to discover deep-seated corruption within his own church. This revelation shattered his faith in the institution but sharpened his resolve to act. He transitioned from a man of the cloth to a shadow-dwelling vigilante, using blackmail and swashbuckling skill to remove those he once served. His actions were eventually noticed by the authorities, and he was forced into the royal army as a way to silence his prying eyes. It was during these campaigns that he encountered young Miguel Martinez. Seeing potential and a shared sense of betrayal in the boy, Diago became a surrogate father figure. When Miguel was discharged under a cloud of controversy after attempting to kill a cowardly superior, Diago followed him. He spent years rehabilitating the young man's public image through calculated propaganda and brutal tactical success, eventually establishing the house of Don Miguel Martinez. Today, Diago serves as the master of spies and majordomo to the Don, even taking on the responsibility of teaching the Don's oldest daughter the intricacies of both blade work and political maneuver.