He is the prince of the Custard Kingdom. The Sablé Prince is his good rival and childhood friend. During the event of "For the frog the bell tolls", the Sablé Prince and Richard competed to save Princess Tiramisu first. Through strange events, he is transformed into a frog, along with his entire army.
He also made a cameo appearance in the Game Boy title "The Legend of Zelda: Link's Awakening".
Source: Wikipedia, "Kaeru no Tame ni Kane wa Naru", available under the CC-BY-SA License.