The keeper of the Hall of Prophecy was a witch or wizard who worked in the Hall of Prophecy in the Ministry of Magic's Department of Mysteries. The known roles of the job entailed labelling records and keeping it up-to-date.
In 1980, the then-keeper of the Hall labelled Sybill Trelawney's first prophecy and put it in the Hall. At first, they labelled it "S.P.T to A.P.W.B.D" to record who gave the prophecy and to whom they gave it, and then put "Dark Lord and (?)" to record who it was about. At the time, it was unknown that the second subject was Harry Potter.
However, in November 1981, the keeper added the name: "Harry Potter" next to ("?"). Albus Dumbledore noted it was "plain to the keeper of the Hall of Prophecy that Voldemort could only have tried to kill [Harry] because he knew [Harry] to be the one to whom Sybill was referring", indicating the keeper had a certain degree of power to determine who they thought each prophecy was about.
- Harry Potter and the Order of the Phoenix (First appearance)