Dr. Halsey's personal journal
Halsey bought the journal at a Reach shop on August 2510. She kept it with her for the next four decades to document her research to counter the Insurrectionist, and later, the Covenant threat. She also expressed her thoughts on her personal relationships with Jacob Keyes and their daughter, her Spartans, and the apocalyptic Human-Covenant War. Halsey eventually left the journal behind at the Babd Catha Forerunner complex, where it was retrieved by Office of Naval Intelligence personnel just before the site, along with SWORD Base, was destroyed.
At the conclusion of the war in 2553, various ONI personnel and psychologists read and analyzed the journal. Based on information from the journal, AI Black-Box suggested that Halsey was trying to cleanse her conscience by hiding her Spartans on Onyx. Both Captain Serin Osman and Naomi-010 also read the journal, and Osman subsequently came to despise Halsey for her extremely unethical work on the SPARTAN-II Program. When finally meeting Halsey on Ivanoff station, ONI director Admiral Margaret Parangosky remarks that she was impressed by Halsey's abilities as an artist, referring to the plentiful sketches she had drawn throughout her journal.
In the journal, several important revelations are made regarding Cortana and Halsey's relationship with Captain Jacob Keyes and Miranda Keyes. Halsey states in an indirect fashion that she and Captain Keyes had a relationship and that though Halsey had been close to Miranda, they had drifted apart. It is strongly implied that Halsey is the mother of Miranda Keyes.
In November 2524 Halsey wrote that she was reunited with "her lieutenant" while attending a conference at the University of Calippus, from which it is inferred that the two became intimate. Four months later she wrote a small side note stating "she will have to answer for making me drink decaf," suggesting Halsey was pregnant. In September, 2525, ten months after her reunion with Keyes, she wrote about experiencing separation anxiety, noting "I guess not hearing her cry wakes me up at night too." Then, on July 12, 2531 on the side of the page, Dr. Halsey wrote, "Jacob has agreed to take her in. I'll miss her, but it's for the best."
Furthermore, it is stated that when Miranda received command of her first ship, UNSC In Amber Clad, she had invited Halsey to attend the ceremony but Halsey refused, stating, "I didn't go. Because her father would be there? Because I am neck deep in the final stages of a half-dozen projects? I know why I didn't go. I don't want to say it." Another hint toward Miranda's parentage is that Halsey wrote in the journal that Miranda had her surname changed to "Keyes", which she believed was done to try and impress her father. It is also mentioned by Halsey that Cortana's appearance somewhat resembled Miranda. Since Cortana was derived from Dr. Halsey's own brain, this remark would not have much sense or relevance had Miranda not been biologically related to her.
The journal also provides an explanation of why Cortana was seen on Reach as well as mentioned in Halo: The Fall of Reach, assisting John-117 and his team of SPARTANs on their mission. It seems Halsey needed to analyze the Forerunner symbols and systems in Professor Sorvad's site. Halsey and Cortana both independently figured out the "Solomonic solution" which helped "divide" Cortana and allow "parts of her" to help Halsey while updating her "incarnate" self on the UNSC Pillar of Autumn until the last minute. The two parts were reunited sometime after Cortana's fragment was delivered by SPARTAN-B312 to Captain Keyes before the Pillar of Autumn left Reach.
It provides an explanation for the inconsistent numbers of Spartans, particularly involving those not present during the Operation: RED FLAG briefing. Halsey states, "That lie about all the Spartans gathered here, save for Gray - will that come back to haunt me?" Halsey notes that the bodies of the candidates who were killed during the augmentation procedures were placed in cryo-stasis, with the possibility that they may some day be resuscitated, though they appeared to be buried in space. She also notes that at least some of the candidates who were crippled could some day be rehabilitated and sent to active duty.
The journal also confirms that Dr. Halsey was aware of the existence of SPARTANs who were "not [hers]" well before the events of Halo: First Strike. She and Kalmiya had uncovered numerous ONI documents and encrypted files that suggested the existence of another SPARTAN program. After the UNSC Hopeful was deployed outside the Epsilon Eridani system for a week, carrying various "carcinogenic", "mutanogenic", and "experimental" drugs. Upon his return, Admiral Ysionris Jeromi informed her that he knew little of what was going on, and could tell her nothing. Finally, Halsey reveals that her suspicions of other SPARTANS were confirmed when she met Noble Team at Sword Base. Later, at CASTLE Base, she discovered that they, with the exception of Jorge-052, had been SPARTAN-IIIs.
 Behind the scenes
The booklet is included with the Limited and Legendary editions of Halo: Reach. The journal was co-written by Eric Nylund in conjunction with Bungie and 343 Industries, with Lorraine McLees serving as art director.
- The journal is missing two pages, of which the date of removal is unknown. Halsey supposedly tears these two pages out after her failed attempt to create a fractal AI housing within Slipspace, presumably to ensure secrecy.
- Halsey's journal can be found on the level "The Package" in the Tribute Room.
- Vice Admiral Michael Stanforth is referenced throughout the journal as "the Vice Admiral", but he is never mentioned by name.
- In the back of Halsey's journal is a built-in folder that contains various documents including a diagram of the Keyes Loop, a blueprint of the MJOLNIR Powered Assault Armor Mk. IV variant, a map of the planet Reach detailing possible artifact dig sites, a security screenshot of Noble Team, and more items that unravel some of the mysteries behind the Halo universe.
A letter from the journal's sender, a former Beta-5 Division operative, to their cousin.
A page of the journal, detailing the SPARTAN-II Augmentation Procedures.
Another page, with information on the creation of Cortana.
The journal as featured in Dr Halsey's lab.