After Horace and his other allies dug pits around the ruins in Antiqua to keep agents of the ruler of Nobilia from stealing from them, Madronius developed a special alchemy formula, Revealer, to allow people to see hidden paths over the pits.
When the Hero came to Horace's Camp seeking the Diamond Eyes, Horace entrusted him to find them and authorized Madronius to teach him Revealer so he could enter the Great Pyramid and the Hall of Collosia. Madronius also informed the hero of his brother in the Hall of Collosia that would also help him.
In the ending, Madronius and Horace are in Nobilia attempting to keep order as Evermore is wracked with earthquakes. The hero comes to get Horace to leave. Horace and Madronius say their goodbyes, and Madronius says it has been an honor to know him. During the credits, Madronius is seen as the new leader of Nobilia.
Madronius gives the player the Revealer formula, and after they collect one of the Diamond Eyes, he gives them the Escape formula. Speaking to Madronius also causes his brother to appear in the Hall of Collosia, who gives the player Fireball.
Madronius sells alchemy ingredients to the player at Horace's Camp. After defeating Aegis, he is no longer accessible. He is the only alchemist to sell Vinegar, the other being Strong Heart once the player acquires the Windwalker, and is also the only alchemist outside of Gothica to sell Brimstone.