Toad

Toads are NPCs that can be found throughout Princess Peach's Castle. Some give helpful tips to complete courses, and some give power stars when talked to. However, if people can get all stars in the Toad Town course, they can unlock Toad as a playable character.

Quin
Quin is a Toad that can have varying colors. When talked to, they will speak gibberish, and can appear 3 more times. Upon talking to them the 4th time, they can alter certain parts of the game.

Dud
Dud is a black Toad that can appear and can cause major anomalous effects.

Toad Projection
Toads start to fade away as you walk away. It is unclear why.

Toads Literally Trapped Inside The Walls
Several characters say that Toads are trapped inside the castle walls. Some people took this literally and theorize Toads were made into the bricks in the walls, similarly to the story of the first Super Mario Bros.