Popful Mail was an old Sega game. Originally, it was going to be called 'Sister Sonic' but the idea was dropped. No one knows for sure why, but likely because the game was extremely dis-similar to anything Sonic. It was done in the anime style, with the heroine Popful Mail. However, as you can see on this town notice board/billboard, it didn't stop them from adding in a Sonic-like portrait paper to give a bit of a shout out.