If any of the players is Matt when in battle, Matt will not show up in the stage background.
Occasionally, Aria may walk by outside the window and look inside for a second before continuing. However, she does not appear if someone is playing as Aria already. Her reaction upon looking inside the window also depends on if Matt is sleeping inside or not.
Matt and Aria plush dolls may appear in the background at random after v1.28 of the game was released. The Matt doll is a bit more common than the Aria one. These dolls are also alternative costumes for Avatar Duo.
There are a few video game disc boxes in the stage. One of them is labeled "The Legend of the Artifact", another other is "Indie Game Battle", and the rest are other 1st party (and some 3rd party) games.