Arthur Trent is the father of Amy Cahill and Dan Cahill.
Arthur Trent went to West Point, and had some classes with Tomas Eisenhower Holt, and laughed at class answers. Years later, Arthur married Hope Cahill and together had two children, first Amy, and then Dan. Their house burned down in a mysterious fire, and Arthur and Hope died, but Dan and Amy escaped the blaze. In A King's Ransom, it is revealed that Arthur may still be alive, and that he is perhaps Vesper 1 (and killed William McIntyre).In the Day of Doom, Dan finds out that his father really had died in the fire and that Isabel Kabra was the one sending him the text messages he thought to be from Arthur.
His father's name is Trent Vesper. Verspers are the only other family that knows about the 39 clues. They are considered evil and Arthur is considered one of them. He was told Cahills were weak little things while he was growing up. After Arthur graduated from West Point, he was told to find Hope Cahill and force her to love him, but once he met her, he fell in love. The ways she laughed, cried, and everything else was irresistable, so he married her. The 39 Clues were extremely important to him, and that is why he went back into the house when it burned down, "supposedly" to save the clue, though the fact that he is still alive suggests otherwise. Also, it is revealed in an Audio Exclusive in a 39 Clues audiobook, that Isabel Kabra's last name before she married was Vesper. Putting the two together, one can infer that she and Arthur plotted against the Cahills and Hope.