Merlon in
Paper Mario
In Paper Mario, he is first seen at his house in the northern plaza of Toad Town. His house is quite obvious as it has a blue spinning roof. It is here, throughout the game, Merlon can give Mario some invaluable information regarding his next mission in the game such as where to go and how to go there, or what a type of adventure the next chapter holds. At a certain point in the game, Merlon can give the quick change badge to Mario if he, while in his house, uses the ultra boots to hit the attic of the house. The badge then falls and Merlon gives it to Mario.
In Paper Mario: The Thousand-Year Door, Merlon lives next to Professor Frankly on the east side of Rougeport. For three of Mario's Shine Sprites he would upgrade one of his team members, giving them more HP and a new ability. With the power of the Ultra Stone, he can upgrade Mario's party even further. The Ultra Stone is obtained by using a similiar method to getting quick change in the first game. This is most likely a reference to Paper Mario 1.
In Super Paper Mario, Merlon plays a more major role. He resides in Flipside and possesses the Light Prognosticus, which was made to counter the Dark Prognosticus. He was the one who found the wounded Timpani and transformed her into a Pixl, Tippi, to save her life at the cost of her memories. Merlon owned the first Pure Heart that was needed to open the first door, to Lineland. He has a parallel version of himself, Nolrem who resides in Flopside.