Who is the middle son of Vito Corleone?

The middle son of Vito Corleone in "The Godfather" is Fredo Corleone. He is portrayed as the less competent and more vulnerable sibling compared to his brothers, Michael and Sonny. Fredo struggles with feelings of inadequacy and often seeks his father's approval, but ultimately his actions lead to significant conflict within the Corleone family.