Antonio is the steward's son and works for the Duke and Duchess at the villa. He delivers letters[1] and helps in the gardens.[2] During the old man's tale, his courtship with the old man's grandmother is mentioned multiple times[3] and the reader is told that Antonio took her as his wife.[4]
He seems a stalwart fellow that helps often and relatively gladly. The Duchess speaks to him directly, suggesting that he is a trusted member of staff. The Duchess also asks for Nencia to call Antonio after the Duke has the statue placed. One can only speculate as to why but perhaps because she assumes he may be able to help either shift the statue or find another way for the trapped Cavaliere Ascanio to escape; this attempt is of course foiled by the Duke requesting to dine with the Duchess immediately after.[5]
[1] see Wharton 1901, p. 7
[2] see Wharton 1901, p. 10
[3] see Wharton 1901, p. 7, p. 11, p. 12
[4] see Wharton 1901, p. 7
[5] see Wharton 1901, p. 16