From Bulbapedia, the community-driven Pokémon encyclopedia.
The Pokémon Mansion journals are a series of diary entries written by a Pokémon researcher, chronicling the events that led up to the creation and escape of Mewtwo. They are scattered around the Pokémon Mansion.
In the games
The Pokémon Mansion journals are only accessible in the Generation I games and Pokémon FireRed and LeafGreen, as the Pokémon Mansion has been destroyed along with most of Cinnabar Island by a volcanic eruption prior to the events of the Generation II games and Pokémon HeartGold and SoulSilver.
Although the English journal entries use "we", the Japanese journal entries only ever use a first person singular pronoun (わたし I).
The Japanese text for this entry reveals the author to be somewhat more emotional.
- Translation of the Japanese text
Diary: September 1
The Pokémon Mewtwo is far too powerful.
It's no use...
I cannot control it!
Relation to Faraway Island
It is speculated that the Guyanese jungle where Mew was apparently discovered is actually Faraway Island. A message is left on a signpost there which contains a remnant of a signature belonging to a name ending in ジ ji but this element is present exclusively in the Japanese release of Pokémon Emerald, which is generally assumed to be from Fuji. The researcher who wrote these diary entries is speculated to be Dr. Fuji, the founder of Pokémon Lab, or Mr. Fuji, who may both be the same character.
In the anime
In Pokémon Origins
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Reason: Add the changes in the texts from the games.
Pokémon Mansion journal in Pokémon Origins
Red found the Pokémon Mansion journal while visiting the Pokémon Mansion in File 3 - Giovanni. Instead of multiple different diaries scattered throughout the building, the journal in Pokémon Origins was a single notebook.
The journal was referenced again in File 4 - Charizard, where Red recalled finding out about the creation of Mewtwo from it. After telling Professor Oak about the journal, Red learned that the journal was probably the notes of "Doctor Fuji", a former Pokémon expert whose research was dismissed soon after he had found a new species of Pokémon and tried to artificially create a new, powerful Pokémon. Professor Oak also realized that "Dr. Fuji" was likely the same person as Mr. Fuji.