Mughi is a large cat-like creature who sometimes accompanies the Dirty Pair on their missions. He has some proficiency with electronic devices, Kei and Yuri usually relying on his help as co-pilot or gun-runner while their spaceship the Lovely Angel orbits the planet they're on, but typically takes a passive role in the Dirty Pair's adventures.
In the anime series, Mughi is depicted as more like a cat or small furry pet than the somewhat threatening creature seen in the novels. Mughi's name has been said to be an acronym for "Military Unit of Genetic Higher Intelligence", according to the character designer.
(Source: Dirty Pair Wiki)