Who is old alberto in this poem ?

The reference may to be Biancha, the 13 year old daughter of Alberto, an admiral of Florence. The story comes from an old elizabethan Play written by Beaumont and Fletcher called 'The Fair Maid of the Inn'. Biancha, the chaste and beautiful girl is the fair maid in the play. Poe may be the referring to the purity of that girl.