Timothy Frisby is the youngest son of Mrs. Frisby, and is a major character in the books Mrs. Frisby and the Rats of NIMH and Racso and the Rats of NIMH. He is described as being thin and small, something of a hypochondriac, as well as being the smartest and most thoughtful of the Frisby children. Early in his life, he was bitten by a spider, which probably accounts for his lack of strength and endurance.
In Mrs. Frisby and the Rats of NIMH, Timothy falls sick to pneumonia, which event is the catalyst for the rest of the story.
Also my favorite character in the NIMH series. Guess where my everything username came from?