Brain is one of Top Cat's loyal followers. Contrary to his name, which appears to have been given in jest or sarcasm, Brain is considered to be the most dim-witted member of the gang. However, he is also in charge of the group's money as treasurer. Despite being the "dumb" one, he can sometimes say something brilliant, which TC instantly rephrases to make it sound like he said it. He is voiced by Leo De Lyon in the original series.
Brain is short, but still taller than Benny, he has deep orange fur (likely an orange tabby) and wears a simple purple shirt with a black stripe at the bottom. His eyes are almost always half-lidded and tired-looking, and he usually has a blank expression, though he can be seen smiling just as the others do, he has a long, thin tail, scruffy cheek fur, and four whiskers (two on each side). In the 1988 film, his fur was a darker orange-brown. In the 2011 film, he as well as the others are digitally shaded, has crooked whiskers, and appears a bit chubbier along with the others, in the 2015 film, his black stripe has been changed to a dark purple.
Brain can be described as being the dull one; he is slow to understand things, clumsy, and naive, but he is kind and well-meaning. Brain is also notorious for being unable to keep a secret and for his stuttering. Top Cat tries to encourage him to think, but when Brain comes up with and idea that TC finds unnecessary, impossible, or just plain dumb, TC often gets irritated with him, though he is never cruel. Brain is shown to be in charge of the money and is designated as the group's treasurer, despite his slowness, he can be surprisingly smart if given the chance. In the 2011 film he seems even more dumbed down further, and his characterization is entirely played for laughs. He also stutters often and is terrible at keeping secrets. In the comics, he is truer to his name, being something of a modern Einstein and knowing many things as an intellectual individual. In the pilot, Hawaii, Here We Come, Brain is introduced in the roll-call, and his establishing character moment is that he forgets what Top Cat was asking him, claiming that he was talking to himself.
Prior to meeting Top Cat, Brain was implied to be a clown working at the circus with Fancy Fancy and Choo Choo. He accidentally let a lion escape while cleaning its cage and thus ran from the circus with Fancy and Chooch and finally met T.C. and Benny at Granny Dibble's house.
- Brain's full name is designated as "The Brain", a term that is almost never used in the show or films.
- His canine counterpart from the 2011 film is Einstein.
- Despite his small size and weak-looking physique, In the 1988 movie and the Kellogg's commercial, he has shown considerable strength, being able to lift Spook and a camera, Rasputin, a large bowl of salad, and an office drawer.
- He is also able to drive, as shown in the final episode of the show and in the 1988 movie.