The "USS Enterprise Officer's Manual" lists Kirk's crew from TOS/TAS, blended with additions from TMP and "Phase II", so it includes Spock, Decker, Ilia, DiFalco, Xon, Arex and M'Ress. Pike is listed separately as a Fleet Captain, now residing on Talos IV - and Number One, referred to only in Pike's biography as "Commander Leigh Chapel", is already off-ship.
DC Comics' first TOS annual included a scene where Number One as replaced as Kirk's intended first officer when her legs are crushed in a cargo bay accident. It was an article of "Star Trek Mysteries... Solved!", in a "Best of Trek" volume, that first speculated on the "Number One is Christine Chapel's older sister" stuff. Writer Leslie Thompson also speculated several possibilities for why Number One had no name, and was rather emotionless.
In my own fanfic, in a story that came out just after ST IV, and set up as a post-ST II story, I called Number One "Admiral Leigh Certaine" (looking up a thesaurus for interesting synonyms for the term "one"), and revealed her to be Christine's sister. I also referred to mysterious alternate names, and mentioned the leg-crushing accident. (It was interesting that Peter David later decided on Morgan "Primus" for his mysterious Number One analog in the "New Frontier" novels.)
Majel Barrett's own take on the character, prepared for her own "research" in the lead-up to making the pilot, was essentially what inspired the DC Fontana take on the character in "Vulcan's Glory". It fits quite well with the idea that Ilyria is an old Earth colony where the populace experimented with an alternate lifestyle.