Theseus was a legendary hero in ancient Greek mythology.
As a young man, Theseus became known for his bravery and strength. He was particularly famous for his role in the myth of the Labyrinth and the Minotaur, in which he entered the Labyrinth, a complex maze on the island of Crete, and killed the Minotaur, a ferocious beast that was half-man and half-bull.
Theseus was also credited with the unification of Attica, a region in ancient Greece that included the city of Athens. He is said to have brought peace and prosperity to the region by consolidating the scattered villages into a single city-state, with Athens as its capital.
In addition to his mythological accomplishments, Theseus was also considered a historical figure in ancient Greece. He was celebrated as the founder of democracy in Athens and was revered as a hero and a symbol of unity for the people of Athens.
Theseus as an Argonaut
Theseus is not generally included as an Argonaut.
In one prominent version of the myth, Jason tells his wife, Medea, the story of Theseus and his wife, Ariadne. It is a story within a story that foreshadows the relationship of Jason and Medea.
In other versions of the myth, Theseus is briefly mentioned to have joined the Argonauts during their journey. He is depicted as a young and inexperienced hero who becomes part of the crew, but his role in the expedition is not significant or well-documented.
Apollonius claims that Theseus and Pirithous were trapped in the underworld by Hades at the time and could not join. Theseus being on the list is inconsistent with accounts of his life usually including him encountering Medea at an early stage of his adventures, yet many years after the Argonauts completed their adventure. Medea, by that time, was not only abandoned by Jason but also bore a child from Aegeus.
More about Theseus on Wikipedia.