Magic brush

In Super Mario Sunshine Bowser Jr. had this paintbrush and said later that a strange old man in a white coat gave it to him. This could mean that Professor E. Gadd gave it to him when he was Shadow Mario and that made the professor think that he was the real Mario and gave it to him. After the great battle on Corona Mountain Bowser Jr. dropped his paintbrush and Il Piantissimo found it on the beach which leads people to believe that he is now Shadow Mario. The paintbrush also appeared in Mario Power Tennis, and somehow Bowser Jr. got it back.