Arnold is the Holt's pit bull.
Arnold was bought by the Holt family, Eiesenhower, Mary-Todd, Hamilton, Reagan, and Madison. They brought him around everywhere, to some week-long winter holidays at the Cahill estate, owned by Grace Cahill. After Grace died, Arnold went to the funeral, falling in line with the Holts. Arnold followed the Holts to the will-reading, trailing Amy and Dan (the mansion fire and museum bomb), and to Pairs. There after the Holts got their ice cream van Arnold played tug-of-war over ice cream with Madison. He stopped when Eisenhower announced that Amy and Dan Cahill had escaped with the Poor Richard's Almanack. Arnold was present when the Holts reached the Île Saint-Louis, and when the thugs arrived to bury Amy and Dan in cement. Arnold grabbed on one's leg, but was knocked unconscious with a misfired concussive grenade along with his owners. The Holts regrouped, and they drove the ice cream van to the St. Pierre de Montmarte church, Arnold barking. They later fled when the police arrived.
While the Holts were on a French train and in Dijon it is unclear where Arnold was. Arnold was next seen in a van the Holts had in Tokyo after they'd captured Amy, Dan, and Alistair Oh. They drove to the Tokyo subway system where it was supposedly the secret network the yakuza controlled. As they stepped on the tracks, Arnold chased after a soot-covered rat, following the Holts, running when a train began speeding toward them.