Justin may be the Captain of the Guard for the rats of NIMH, but don't let the title fool you; he's not just a soldier.  He's a rat of impeccable character...kind, noble and friendly.  And yet he's also a skilled leader who has proven himself time and again to Nicodemus and the other rats, and is held in high regard as a result.  And, when necessary, he's pretty handy with a sword!  When Nicodemus meets an untimely end at the hands of Jenner's schemes, it is Justin who rises to the leadership of the rats of NIMH.

Quote: "We tend to take ourselves a little too seriously."

Origins of the character: In Robert C. O'Brien's book "Mrs. Frisby And The Rats Of NIMH", Justin is pretty much the same character we see in the movie (mostly).  He is described, when Mrs. Frisby first meets him as "...alert, dark gray in color, and extraordinarily handsome, though not so huge as Brutus."  Justin is never officially given the title he has in the movie; though, in the book, Nicodemus does admit to Mrs. Frisby that "...indeed, you might call Justin the captain of the guard--if we had any such titles, but we don't."  Justin is definitely, on the other hand, an influential and important member of the rats of NIMH.  In the book, it is both he and Jenner who initially figure out the means of freeing the rats and finding an escape route out of the NIMH laboratory.  In fact, it is Justin who first begins looking for it, which is what brings him to Nicodemus' attention in the first place.  In the book, as much as in the movies, he is a noble and honorable character.

In the animated sequel: There really aren't any changes to this character worth mentioning in the animated sequel.  Not only is he pretty much the same, indeed he hardly seems to have aged at all.  But while he has definitely maintained the leadership of the Thorn Valley colony, it's also evident that he shares many leadership roles, in the sequel, with Mr. Ages, who has apparently decided to move into the colony from his previous home on the Fitzgibbons farm.