Fatty was created by Annette Billingsley as a character in her thirty-two book series of The Troubleseekers children's novels. He was an overweight child who desperately wanted to be part of the Troubleseekers group, but the the children found him annoying, so instead he followed them and tattled on them.
In a world created by fictional books that Martha Jones had read, Fatty was temporarily brought to life by a Ch'otterai. He followed Martha and the Tenth Doctor and reported back to the Ch'otterai what they were doing in his fictional world. (PROSE: The Mystery of the Haunted Cottage)