From the Super Mario Wiki
Flopside is the mirrored counterpart to Flipside in the game Super Paper Mario. Both places' names come from the phrase "flip-flop". Flopside was created by both the Tribe of Ancients and the Tribe of Darkness as an antithesis to Flipside, in order to prevent that town from falling into ruin due to the fact that light cannot exist without darkness. The two towns offset each other, and have been in perfect balance since their creation.
The main features that distinguish Flopside from Flipside are its shadowy tones, a general dilapidated look such as broken windows and pillars, and a reversal of areas, buildings, and residents. Inanimate objects are mirror images of their Flipside counterparts, and the Flopside counterparts of Flipside residents have opposite personalities. For example, Pook of Flipside loves the sea, while his Flopside counterpart, Puck, prefers the sky. Another example is the cook Dyllis, Saffron's counterpart, who mixes two items instead of simply cooking one. Generally, the ambience of Flopside is very dour and disconsolate. Flopside's music is almost the exact same as the Flipside music, lacking in a few notes and transposed down several keys.
Like Flipside, Flopside contains its own Pit of 100 Trials, the boss of which is Shadoo. It also houses half of the Heart Pillars, which are located in the same spot in relativity to Flipside. The Mirror Hall serves as a bridge between the two towns, and is supposedly a secret to all Flipside and Flopside residents aside from Merlon and his Flopside counterpart Nolrem. However, as the game progresses, the player encounters many characters that know about the other town, and one Flopside resident even moves to Flipside.
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