From the Super Mario Wiki
Jack, mentioned only in Mario Party Advance, is the main protagonist of the TV show Toad Force V. He is a robotic hero that fights against the Spore King and his minion Toxic Toad Z in the name of peace. His sidekick is Britney, and a full fighting force of heroes (referred to merely as "Toad Force V") is also implied.
Jack is the only Toad Force V character given a physical appearance, albeit through Toad Force V promotional items, such as posters and action figures. While most images of Jack depict him simply as a shadowed Toad with a "V" on his mushroom cap, the more detailed action figures show him to be a short, white-faced Toad wearing a red shirt with a white star, blue pants, and white shoes with red bottoms.