Owner of the Golden Fleece
King Aeëtes was a king of Colchis, father of Medea, and owner of the Golden Fleece. Although, not a crewmember of the Argonauts, King Aeëtes is a main character in the story of Jason and the Argonauts.
When Jason arrived to claim the Golden Fleece. Aeëtes promised to give it to Jason only if he could perform certain tasks. The first task was to plow a field with fire-breathing oxen. Next, Jason sowed the field with the teeth of a dragon that King Aeëtes had received from Athena. The teeth sprouted into an army of skeleton warriors. Jason was quick-witted, however, and before they could attack, he threw a rock amidst the skeletons. Unable to determine who threw the rock, the soldiers killed each other.
Even though Jason had completed the tasks and proven himself worthy, King Aeetes had no intention of giving him the Golden Fleece. With the help of Medea, Jason stole the fleece. And they both sailed away in the Argo. Aeëtes pursued them in his own ship as they fled, but Medea distracted her father by killing and dismembering her brother, Absyrtus, and throwing pieces of his body overboard. Aeëtes paused to gather the pieces of his son, and thus Jason and Medea escaped.
Read more about King Aeëtes on Wikipedia.