Nien Nunb is a Sullustan, a member of the jowled, rodent-like race that hails from the planet Sullust. The toxic gases of this world make the surface uninhabitable, but the Sullustans live in a gigantic network of caverns beneath the surface that covers the entire globe. This environment has made Sullustans' eyes much more sensitive to light than humans' eyes. They can see in nearly pitch-black, but are uncomfortable in full daylight. They have also developed a keen memory to prevent them from becoming lost in their maze-like tunnels. Once a Sullustan has gone somewhere, he can always retrace his steps there again. This makes them extremely valuable as copilots and navigators.
The planet isn't ruled by a normal government; instead, the megacorporation SoroSuub makes and enforces the laws. SoroSuub employs over 90% of the planet's inhabitants and is a premier producer of high tech goods such as starships, blasters, and luxury landspeeders.
The SoroSuub Company was fiercely loyal to the Empire during the classic Star Wars era. The majority of the Sullustan population was sympathetic to the Rebellion, however, which caused great friction between the two groups. It wasn't until the heroic Nien Nunb, however, that the planet openly rebelled. He started as a shipper for SoroSuub and quickly became the fastest and the best of the company. Once he learned of the rebellion, he quickly switched sides and began pirating SoroSuub goods.
This elevated Nien to folk hero status, and it wasn't long before the planet was in full rebellion. The Rebel Alliance chose the planet as a gathering place for their fleet before the Battle of Endor, and strike leader Lando Calrissian choose Nien Nunb as his copilot. The Rebel Fleet won with the help of the Sullustans, and the Lando-Nunb duo destroyed the second death star.
The wonderful individuals who deserve credit for bringing our hero to the big screen are Dave Carson, who designed and sculpted Nien Nunb's beautiful features, and Mike Quinn, the puppeteer behind Nien during his renowned piloting scenes.
You can find Mike Quinn's site here. Richard Bonehill is the man behind Nien's Mask during his other scenes. For more information on Richard, check here (special thanks to Matthew Bonehill).