Jerome, usually known as "J," is the narrator who lives and works in London. He is taking a vacation along the Thames river along with two friends. J. is confident and fairly jovial, always ready to exploit the funny failures of his two friends as he tells us, the readers, his story. J. always makes himself seem better than his companions, he always seems to be "above" their immature antics.
George is one of the two friends who accompany J. on his trip along the Thames (for part of the time, anyway, due to his occupation). He is a banker who has a very strict time schedule and who is already ready for a pint after hours. George is quite a character, always boisterous. When he shows up for the vacation, he is sporting his wild style and carrying a banjo. No one quite knows why George is carrying this ethereal banjo. He can't play music. Therefore, despite his quirks, J. still considers George quite boring except for his ability to find pubs.
William Samuel Harris is the other of the two friends who join J. on the vacation along the Thames. J. finds William incredibly boring, lacking all "romance and poetry." William is a lot like George in that they both like to plan their pubs and eats. William also loves music, but has a horrible memory so can never remember the words to the comedy songs he loves so much. There is one significant time on the trip along the Thames that William remains on the boat only to get exceedingly drunk. William is also the one to propose the toast after they get off of the boat. (Due to heavy rains, the three decide to take the train instead.) "Here's to Three Men well out of a Boat!"
Montmorency, although a dog, is quite a character. He is a crew member of the boat along the Thames. J, George, and William swear continually that Montmorency is always fighting and would fight any other dog anywhere. Ironically, the only thing that can make this dog retreat is a cat! Apart from all this, Montmorency is always the most realistic of the four.