Dr. Halsey's personal journal

Dr. Halsey's personal journal is a booklet co-written by Eric Nylund and is included with the Halo: Reach Limited and Legendary editions. Not much is known about it, other than it covers various things in the Halo Universe and is written through the eyes of Catherine Halsey herself.

Description
It is in this journal that several important revelations are made regarding Cortana, Her relationship with Captain Jacob Keyes and Miranda Keyes. Per the journal, she states in an indirect fashion, that she and Captain Keyes had a relationship and that though Halsey had been close with Miranda, they had drifted apart.

Furthermore, it is stated that when Miranda received command of her first ship, In Amber Clad, she invited Halsey to attend the ceremony but Halsey refused stating that "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". From this it is inferred that Halsey is the mother of Miranda Keyes. Another clue to this theory 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.

The journal also provides an explanation of why Cortana was seen on Reach as well as mentioned in Halo: The Fall of Reach in 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 Pillar of Autumn until the last minute. The two parts were reunited sometime after The Package (level) was delivered by Noble Six to Captain Keyes before the Pillar of Autumn left Reach.

It also provided an explanation for the inconsistent numbers of Spartans particularly involving MIA's during the "Red Flag" briefing. Halsey states, "That lie about all the Spartans gathered here, save for Gray - will that come back to haunt me?"