Gen the Puppy is a former abandoned dog that was taken in by Aoi Shinagawa. He is named after Gen Shishio, who fed him while he was homeless.
He is a very small dog (likely a beagle) with a tan coat, though his ears, feet, and tail are all dark brown. His right eye is also surrounded by a dark brown spot.
On his way to school one morning, Gen Shishio comes across a sad, abandoned puppy in an alley. Feeling sorry for it, he gives the puppy part of his burger to eat. Aoi watches this from the other end of the alley. Yurina Kanda later visits Aoi at home, and is surprised to see the puppy there. Aoi explains how she saw Gen feeding the puppy, and since it was still in the alley on her way home, she decided to keep it. After Aoi is rejected by Gen, she decides it is for the best, and is happy that because of him, she was able to meet her puppy, which she decides to name after Gen. She mentions that she has already taught him to sit up, beg, and shake hands, and considers training him to be a ninja hound (because she thinks Gen is a ninja descendant).
Gen the Puppy is the third character in the series to have the name "Gen," with the others being his namesake Gen Shishio, and the ghost Gen-san.