Who is Marchbanks?

Eugene Marchbanks is one of the main characters in the play alongside Candida and her husband Morell. Candida explains who he is soon after arriving back home. She tells her father and her husband that Eugene is a nobleman who her friend James discovered sleeping on the embankment. She describes him as a shy, "dear boy" and has brought him back to stay with them.
When he enters the scene a moment later, Bernard Shaw describes him as follows:

He is a strange, shy youth of eighteen, slight, effeminate, with a delicate childish voice, and a hunted, tormented expression and shrinking manner that show the painful sensitiveness that very swift and acute apprehensiveness produces in youth, before the character has grown to its full strength.

From the beginning, Bernard Shaw portrays Eugene as a very nervous young man. When Morell introduces him to his father-in-law, Eugene nervously backs himself against the bookcase.
Eugene admits to Morell that he is in love with his wife. At first Morell seems alright with it, saying "everybody loves her: they can't help it", but he becomes increasingly uncomfortable with it as the play wears on. In the end, Eugene asks Candida to choose between him and her husband. She chooses her husband because she thinks he needs her more.