Duke Gromgard is the ruler of Greenvale, the father of the Lord Gromgard, Gerda, and Grenville, and the nephew of the Black Baron, the previous ruler. He only appears in the opening and ending cutscenes.
When his land's prosperity plummeted, the Duke traveled to foreign lands in a bid to bring back wealth to restore his country's former glory, only for his wife to leave him for a strong, rich nobleman from another realm. To make matters worse, the Duke returned from his quest empty-handed. Broken-hearted and consumed with his obsession to restore glory to Castle Gromgard, the Duke set out on another quest on Lord Gromgard's 16th birthday, leaving his youngest son in the care of his abusive older siblings. After the Overlord takes over, the Duke seems happy with the new order of things and decides to continue his quests for fortune and it is suggested by Gnarl that he is also seeking a new wife.
In the ending cutscenes, he is seen to be quite the troublemaker. Some of his misadventures including upsetting barmaids by touching them inapropriately, riding a Unicorn while facing the wrong way, being chased by an angry farmer wielding a pitchfork, partying with several villagers only to be punched by one of their wives again (possibly for touching her inapropriately) and, finally, standing on a box to whisper in the ear of a rather large (as in stocky, not fat) barmaid, who is later seen carrying him bridal style down the steps of Castle Gromgard, implying they eventually married. The barmaid seems to be Doris the Witch.
He also appears to be aware of the Minions, as several Browns are seen in the ending cutscenes with him, though whether he knew of them from his uncles's time, or Lord Gromgard sent them to accompany his father on his journey is unknown.