Star Trek: 10 Things You Didn't Know About Deanna Troi
9. Character Constructs: Not A Total Recall
What you may not know is that the character of Troi first took reference from prior Trek, and Ilia in particular from The Motion Picture (think DELTA-ns and BETA-zoids and their mental capabilities). The latter was already a carry-over from the abandoned project Star Trek: Phase II. Ilia’s relationship with Will Decker was then the model to build that between Troi and Riker.
Both Marina Sirtis and Denise Crosby also originally considered Troi to be "Spock-like". The character description for Troi in the TNG Series Bible does make passing reference to Spock, but this is merely to flesh out the counsellor’s telepathic abilities. Troi was, ultimately, meant to be her own person, her role nonetheless "second in importance only to the captain and first officer" according to the Series Bible once more.
There seems to be some confusion surrounding the original intent for the character, however. Between conflicting descriptions of Roddenberry calling Troi both "the brain on the show" and "sexy and not very bright," it is difficult to know what to think. Apparently, Roddenberry initially wanted Troi to have three (or even four depending on the source) breasts before D.C. Fontana THANKFULLY convinced him otherwise. Sirtis would later note that, with the costume changes of season one, "[…] when [Troi] […] got cleavage, she lost her brain matter", and only really gained some back when she (re)gained the uniform.