Sir Colin Bartholomew Montgomery Rothchild III (shortened to just Rothchild) is an ally of Samurai Jack and part of a group called the Canine Archaeologists. He was introduced in Episode II: The Samurai Called Jack.
Similarly to the archaeologists, Rothchild resembles his quadrapedal ancestors in size, appearance and certain characteristics, such as wagging their tails. Rothchild and the archaeologists gained the ability to walk upright, use their front paws as hands, complete with opposable thumbs, enhanced intelligence and the ability to talk fluently.
After fifty years, he grows a beard.
Rothchild shares the same personality as his group, the archaeologists, who learn to study their past. Rothchild is an intelligent group and is well mannered, but still retain habits from their ancestors, like riding a vehicle with their heads out the window or wagging their tails when happy.
Rothchild and the canines first appear in the bar Jack came into to find a drink. They were trying to convince an alien to help them fight against Aku in order to free themselves of his grasp. The alien called them idiots and left and so they turned their attention to Jack, who had defeated a large number if aliens without taking a hit. Rothchild and the canines introduce themselves to Jack, and help explain to Jack his current situation, and that they need his help. Jack accepts and the group leaves, though they didn't realize that they were being watched by one of Aku's servants. The servant informed Aku and Aku sends an army of Beetle Drones to destroy Jack and the canine.
As they reach the excavation site, Jack, Rothschild and the canines learn that Aku is sending his forces to obliterate them at sunrise. Jack comes up with a plan to stop the advancing horde, and with the the help of the dogs, sets up various traps and makes some weapons to fight. As sunrise comes, Jack heads into action. Using the traps and his new weapons, Jack manages to wipe out a large number of drones as the Canines watch from their hiding places. Eventually, Jack defeats the last of the drones, freeing the canines and enabling them to continue their research. Rothchild and the rest of the group offer Jack a chance to go with them, but he declines and goes on his way.
Sir Rothchild make his return in Episode CI with multiple grandchildren that resemble him. He and his family witness Aku's triumphant broadcast and immediately assault The Pit of Hate with Jack's other allies using their flying vehicles. Some of his grandchildren were taken down by Aku's corrupted forces, with Rothchild himself being shot down after being damaged by Aku's rain of spikes.
It is unknown how the past Aku's destruction affect his existence and also the canine archaeologists.