In The Red Chamber Dream, Zhen Bao-yu is a very minor character that is mentioned several times throughout the text, and only is actually seen once, near the end of the work.
The importance of this character, of course, is that his name is almost the same as Jia Bao-yu, the protagonist, with the exception that his family name is "Zhen", "reality" instead of "Jia" "false".
His family are literary gentlemen that associate with Bao-yu's family, and they tell Bao-yu's amused family about their own Bao-yu, a lively yet lazy boy who enjoys girls more then studying. This, of course, totally matches the protagonist of the story. There physical appearence is even said to be identical.
However, later on when the two Bao-yu's finally meet, the main Bao-yu finds out his twin has become a straight and narror parroter of Confucian jargon. The hero Bao-yu is, of course, quite disgusted.
The meaning of this character is quite hard to guess, the narrative would have been quite safe without this interjection, but the names and appearences of the characters does provide an illustration of the novel's philosophic themes, which is the difference between the real and the unreal.
Of course, it also brings up two questions: Can there be another person that matches our appearence and behavior totally? And is that person really the same as us? And if we met that person, would we like them, or hate them?