Jack

Lives in dog room 701 with Legoshi
As a child, he had a falling out with Legoshi, which led to their friendship.
He sends his family a letter about his condition and well-being at least once a month.
Jack knows more about Legoshi than anyone else. From how Legoshi can silently laugh at a joke he likes all day long, to what it means when he suddenly sniffles on the phone in the middle of the night.
Jack worries that Legoshi is maturing faster than he used to, and that they could soon become very distant because of this. He's a yellow Labrador retriever and has been shown wearing various outfits throughout the manga, but is usually shown in his school uniform.

When Legoshi dropped out of school, he often wore his old clothes that were too big for him.