Most of the original characters which appeared in Robert C. O'Brien's book "Mrs. Frisby and the Rats of NIMH" found themselves, in one way or another, represented in the movie "The Secret of NIMH". All except for three: Paul, Isabella and Arthur.
"The Lost Three"
When Robert C. O'Brien wrote his excellent book, Mrs. Frisby and the Rats of NIMH, he populated the story with several richly-designed characters, almost all of whom ended up being developed by Don Bluth and his team for the animated adaptation of O'Brien's book, The Secret of NIMH. However, any fan of the book will recall that, of the many characters which were adapted for the movie, three of O'Brien's were left out of the story. And while none of them were technically essential to the movie adaptation (though, as fans, we never have been given an explanation as to why that was, nor why they never appeared in it), they are nevertheless part of the NIMH canon, and so I would like to share them with you here, in the event you have not read the book and would like to get to know them. And certainly if you would like to use them for your own fan fictions, fan art, etc.
Here they are, with descriptions (in the quotations) of each taken from O'Brien's book:
Above: In a drawing from O'Brien's book, Mrs. Frisby watches from a hole in a fence post while Mr. Fitzgibbon and his son Paul prepare the tractor for plowing the field.
When we get to meet the Fitzgibbons family in The Secret of NIMH (remembering, of course that, in the movie, their name is spelled with an "s" on the end, where it is not in the book...another name change by Bluth and his team), we are introduced to three people: Farmer Fitzgibbons, his wife Mrs. Fitzgibbons (whom the farmer, at one point in the movie, calls "Beth"), and their young son Billy Fitzgibbons. We never get to know how old Billy is, but it is clear that he's most likely a preteen...assumedly about 10-12 years of age. In the book, Billy Fitzgibbon is described as being 12 years of age. However, one member of the family is missing: the Fitzgibbons' older son Paul.
Paul is a teenager, and a much different boy than his younger brother. He is described as being 15 years old, and "was a quiet, hardworking boy, rather clumsy in his movements but strong and careful about his chores." He comes across, in the book, as a serious and intelligent fellow, with a sympathy for the animals which his younger brother, Billy, enjoys capturing or tossing rocks at. And he proves to be a helpful hand around the farm.
Above: In a drawing from O'Brien's book, while waiting in the library, Mrs. Frisby startles a young female rat named Isabella (on the right), who walks in on her unexpectedly.
In the midst of the story, as Mrs. Frisby visits the rose bush colony of the rats of NIMH, she is escorted to the colony library. Even in The Secret of NIMH we hear Mr. Ages ask Justin to take her there. Though, in the book, Mrs. Frisby is not brought first into the council chambers, but rather directly to see Nicodemus for an initial introduction and meeting--after which it is Nicodemus himself who asks Justin to escort her to the library to wait while the rats and Mr. Ages attend the council meeting. While there, Mrs. Frisby studies some information scrawled onto a chalk board and explores the room. While she is doing this, the door to the library opens, and someone walks in.
"It was a girl-rat, small and quite young, judging by her looks. She was carrying a pencil and some papers and looking at the papers as she walked, so that she did not see Mrs. Frisby at first. When she did she gasped and dropped the papers, scattering them on the floor. Her eyes opened wide."
Isabella nervously questions Mrs. Frisby as to her identity, thinking she might be a spy from NIMH. After Mrs. Frisby assures her that she is not a spy, the two have a conversation in which we learn a good deal about the young rat. She was not born until after the rats had arrived and established the colony under the rose bush, though both of her parents came from NIMH. She also reveals that she has quite a crush on Justin, and considers him so brave that not even Dragon, the Fitzgibbons' cat, could rattle him. But she's concerned that she's too young to marry, and that she might lose Justin to another girl rat.
In the sequel book Racso and the Rats of NIMH, Isabella appears again, though a bit older (approximately the age of a human teenager, comparatively), and still has a hardcore crush on Justin, which we learn is utterly unrequited. It frustrates poor Isabella, who keeps trying to impress him, and she is devastated when she sees him fall in love with another rat. Though, in the end, she does find a new love interest.
Still, it might have been interesting to see how Isabella's affections would have created difficulties for Justin and the other rats, and Mrs. Brisby, had she been included in The Secret of NIMH!
Above: In a drawing from O'Brien's book, several of the rats of NIMH are held at bay by the shrew, who thinks they are at the Frisby home to steal from it. As Justin tries to explain their purpose for being there (and Mrs. Frisby looks on), we see a rat nursing his leg, which had just been bitten by the shrew. This is Arthur.
After Justin, Nicodemus and Mr. Ages return from having attended the council meeting, in the rose bush colony of the rats of NIMH, they reenter the library to talk with Mrs. Frisby. Accompanying them is another rat, whom Mrs. Frisby had not seen before.
"The strange rat was named Arthur. He was stocky, square and muscular, with bright, hard eyes. He looked efficient." Nicodemus introduces him to Mrs. Frisby as the rats' chief engineer, whom Mr. Ages felt should be present in the discussion about moving the Frisby home. Arthur proves to be important to developing the method the rats use in moving the Frisby home (which, in the book, they complete successfully, without any interruptions in the form of villainous scheming by one of their own, or the intervention of a magical amulet).
Arthur appears a few times in the book, and then in the sequel literature, and is a key member of the rats of NIMH, with a very important job. It is odd that he, especially, does not appear in The Secret of NIMH.