Brain, as well as Pinky, is a white mouse who is being kept as part of Acme Labs' experimentation. They have undergone significant genetic alteration; as per the show's title lyrics, "their genes have been spliced" which gives the two mice amplified intelligence over that of a typical mouse.
Brain looks and sounds like Orson Welles, albeit with pink eyes. In "What Ever Happened to Baby Brain," Brain in fact crosses paths with Welles, who is working as a busboy in a Hollywood restaurant; they find themselves inadvertently yelling in unison, "Things will be different when I take over the world!"
In "Project B.R.A.I.N." Brain's name is the acronym for "Biological Recombinant Algorithmic Intelligence Nexus."
His tail is bent like a staircase—he often uses it to pick the lock of his cage—and his head is large and wide, housing his abnormally large brain.
He is highly intelligent and develops complex plans for global domination (only nobody believed he will), using politics, cultural references, and his own inventions toward his goal. He seems coldly unemotional, speaking in a deadpan. Nevertheless, Brain has a subtle sense of humor and even possesses some Christmas spirit. He has even fallen in love, with Trudie in the episode "The Third Mouse", and with Billie in "The World Can Wait." Brain has been compared to Napoleon Bonaparte and Don Quixote.