Tatsuya Matsumoto is David Read's old pen pal from Japan that he wrote to in seventh grade. He was first mentioned in "Dear Adil," where his old letters motivate Arthur to get a pen pal. He was first seen in a brief appearance in "Flea to Be You and Me," where he played an instrumental role in unknowingly getting Pepe to America. In this episode, he sent David Read rice vinegar. Although it is likely not him, a character reusing his design appears in the movie in "The Squirrels."