Despite what many might believe, behavior and personality were not unconditionally linked. Such was the case with Desmond, who tended to be very careful and cautious, but whose nature tended to be more carefree. Because of this, Desmond grew to show great focus on some issues but be careless on others; hence he will continue to be relatively ignorant of certain things around him, Estela included.
Estela was a very peculiar lifeform; even now, having visited half a dozen worlds, Desmond had only seen two non-organic lifeforms, including Estela. Unlike the android that Desmond had known, Estela was literally a creature made of minerals that were given life under who knows what strange logic.
Even now, Desmond still didn't know much about what Estela was and what she could do; perhaps he never bothered to investigate too much, given the nature of Estela's position within the household.
