Brutus is a hulking rat who serves as a guard at the entrance to the rats of NIMH colony, established within (and under) the rose bush on the Fitzgibbons farm.  He's the strong, silent type in the first movie, never uttering a word, but presenting a very intimidating and ominous presence before the entrance to the colony.  He carries a large, elaborately-bladed halberd which somehow manages to shed an electric charge when it strikes anything solid.  And while he's a formidable presence, effectively frightening Mrs. Brisby nearly out of her wits, he clearly doesn't mean her any real harm, but is satisfied in just trying to scare her away.

Quote (from the sequel): "Yo!  What in the name of Nicodemus?!"

Origins of the character: In Robert C. O'Brien's book "Mrs. Frisby And The Rats Of NIMH", Brutus is a more active character than in the movie.  And he's rather more talkative.  While he holds no weapon on guard in the book, he is described briefly as Mrs. Frisby stumbles upon him: "beside the entranceway, looking at her with dark, unblinking eyes, stood the biggest rat she had ever seen."  Brutus doesn't frighten her or chase her off, but he does bluntly and rudely order her to go, under threat of being carried out.

We learn, however, that he's got a heroic side to him.  Near the end, when the rats are leaving for Thorn Valley, ten of them (including Brutus and Justin) stay behind to try and mislead the NIMH scientists, and make them believe that they are just ordinary rats.  When the scientists flood the colony (which has been disassembled and made to look like a normal rat nest) with cyanide gas, the rats flee and get away.  Brutus, the last to leave, stumbles out of an escape hole and collapses, apparently overcome by the gas.

In the animated sequel: When we next see Brutus, in the movie sequel, he has gone through a significant redesign by the artists and animators, and also has a voice actor (and an active role in the movie).  Brutus is still a big, strong and heroic rat, and now he appears to serve the rats of NIMH in relatively the same fashion as Justin did in the first movie.  And while he's not the brighest bulb in the box, he's got a good heart and is absolutely fearless.  He takes good care of young Timothy Brisby during the adventures they have together.  Below we see Brutus and young Timothy Brisby in the midst of an adventure.