Jonathan Teatime (TEH-ah-TIM-eh) is a character from Hogfather, part of the Discworld series of books by Terry Pratchett. He is a student at the Ankh-Morpork Assassins' Guild. He was taken in by the Guild after he lost both parents at an early age; Lord Downey, head of the Assassins' Guild, has remarked that they should perhaps have wondered more about this. Due to a childhood accident, he has only one real eye; the other is made of glass.
Mr. Teatime is most famous for the attempted assassination of the Hogfather, who is a sort of Discworld version of Santa Claus. The attempt is especially famous because of the amount of money involved ($3,000,000, three times the highest price the Guild had ever set on anyone), as well as the nature of those hiring the Guild – the trans-dimensional Auditors of Reality.
The method of attack on the Hogfather was remarkably simple. Beings such as the Hogfather "exist" on Discworld due to the amount of belief placed in them, and their power is proportional to this belief. Mr. Teatime therefore used the simple magical principle of using a part of something to control the whole; he stormed the castle of the Tooth Fairy to gain access to the teeth of children all over the Disc, which he used to make them not believe in the Hogfather.
This would have worked, except for the efforts of Death, who took over the Hogfather's duties in order to maintain some belief, and Susan Sto Helit, Death's human granddaughter (it's a long story), who took out Mr. Teatime's assistants, recovered the teeth, and saved an assistant Tooth Fairy who had been kidnapped. Mr. Teatime himself was eventually defeated by Susan and Death working together.
Sources:
Pratchett, Terry. Hogfather. 3rd ed. New York: HarperCollins, 2000.
Pratchett, Terry, and Stephen Briggs. The Discworld Companion. 2nd ed. London: Orion Books, Ltd, 1997. pp. 379-380.