He was St. John.

The New Testament states: "Standing by the cross of Jesus were His mother and His mother's sister, Mary the wife of Clopas, and Mary of Magdala. When Jesus saw His mother and the disciple there whom He loved, He said to His mother, 'Woman, behold, your son.' Then He said to the disciple, 'Behold, your mother.' And from that hour the disciple took her into his home." (John, xix, 25-27).

Also, see links off this page: http://www.apostles.com/stjohn.html