From the Super Mario Wiki
Mushbert is a Toad only appearing in the game Mario Party Advance. He wears a green mushroom cap. Mushbert loves to find collectibles about the hit TV show Toad Force V. He can be found on the first floor of the Mushroom Condo. Once the player enters his apartment without being on a quest, he will force them to leave, saying that he is busy.
The quest DVD For Me involves going to Mushbert's apartment to get a rare Toad Force V DVD for Ninji. However, he didn't mind giving one away; he had forty-nine more. After completing the quest, Ninji states that only fifty were made, meaning Mushbert had bought them all.
The quest What's That Line? involves going to his apartment to learn the famous quote from episode 28 for Lakitu.
At the end of the game, it's said that he outgrew Toad Force V and started watching Koopa Quest.