Castor (Once a Hero)

Castor of Argos was a famed Argonaut, noted for his acrobatic skill. He attempted to become the King of Argos. (HTLJ: "Once a Hero")

Castor was amongst the crew of the Argo when the golden fleece was found, although he didn't meet Hercules until after the voyage. (HTLJ: "Once a Hero"; YH: "Winner Take All")
 * The voyage, shown onscreen in Young Hercules didn't feature Castor in any capacity.

In the ten years after the voyage of the Argonauts, Castor gained a hatred of Jason's success. After Jason's family was killed by Medea, Castor impersonated a demon and appeared to Jason, driving him into alcoholism and madness.

Castor attended the Argonaut reunion, and was amongst the crew on the second journey of the Argo to seek out the golden fleece (which he had stolen with Marcus's assistance). He secretly killed the Argonauts Lycenus, Valerus, and Otus, before "regaining" the fleece and proclaiming himself as King of Argos. Upon Jason's suggestion, he fought Jason for the throne and was impaled on his sword. (HTLJ: "Once a Hero")

Background information
Castor was portrayed by Peter Feeney.

Although similarly named, Castor doesn't bear any relation to the mythological Castor, although another character does.