The Trent Family is not to be confused with the original Cahill Family. Dan and Amy's father's last name is Trent, but he chose to accept his wife's maiden name of Cahill for their children's last names. This may have been because Madrigals usually take their mother's name instead of their fathers in honor of Madeleine Cahill. Arthur Trent was a Vesper sent to spy on and capture Hope Cahill but fell in love with her and betrayed the Vespers to become a Cahill and marry Hope Cahill. Arthur Trent was the only non-Cahill that knew about the search for the 39 Clues before he married a Cahill. Unfortunately, Arthur Trent was never granted a full Madrigal status. He also for some reason, kept the last name Trent instead of changing his name to his wife's maiden name but there are mostly Madrigal