The Toothbrush is first seen at night, appearing on the floor. Telly suggests that it is able to be obtained and could be used to clean stains. You can earn Happy Points for doing so. There are stains like mud from footprints, pawprints, puddles, tears, and oil.
The Toothbrush returns as one of the main cleaning tools in this game. It will be obtained by collecting it from the rim of the sink found in the bathroom. You will most likely need Telly for this access, however. By calling him from the platform, you can fly over to the sink and obtain the toothbrush. The toothbrush can clean substances such as dirt, water, crayon marks, footprints, pawprints, and in a mini-chore, Lucky.
Though the toothbrush is not able to be used in this game, it briefly appears in the first cutscene when Chibi-Robo uses it to clean the ship.