The Mouth of the River, also known as Madame Mouth or simply The Mouth, is a gypsy-like woman who inhabits the River World, which she knows everything about.
In The Super Mario Bros. Super Show! episode "Rolling Down the River", Mario, Luigi and Mark Twang seek the advice of the Mouth of the River to help them pass the black pit, which blocked the path of Mark Twang's ship, the Ding Bell. Finding the Mouth of the River's caravan, Mario, Luigi and Mark Twang gained some cryptic advice from her, have the river "go around" the black pit.
Later, after the Ding Bell passes the black pit through the aid of a river-making pipe system built by Mario and Luigi, the Mouth of the River joins the crew of the Ding Bell. While onboard the Ding Bell, the Mouth of the River gives Mario and Luigi two magic doodads which, when worn by Mario and Luigi, transforms them into Super Mario and Super Luigi; with their new superpowers, Mario and Luigi easily power the Ding Bell ahead by using a large, jump-powered pump.
The Mouth of the River later appears to attack Captain Koopa's ship, the Sinister Star with a large, turnip-launching cannon attached to the Ding Bell. Luckily, one of the turnips fired by the Mouth of the River's cannon managed to hit the driver of the Sinister Star, Clawgrip, knocking him unconscious and allowing Princess Toadstool to take control of the Sinister Star.
Afterwards, after Captain Koopa is defeated, the Sinister Star destroyed and the Ding Bell crosses the finish line of the River World, the Mouth of the River appears to serve Mario, Luigi, Toad, Princess Toadstool and Mark Twang their victory lunch, catfish pizza, with a raw catfish on it.