I established her in the outline as the character who would become the Enterprise's new contact specialist, with the assumption that she would fill the "third seat" hitherto occupied by Deanna. I received no objections to that idea, although her role has been subtly redefined by subsequent authors.
I think there's a false underlying assumption at play here, in terms of the use of the phrase "regular character." That's a term from television, referring to a cast member who's contracted to appear in most or all of a season's episodes, as distinct from a player who's hired on an episode-by-episode basis. It doesn't really apply in novels, because there are no actors involved. We introduce characters and see what happens. T'Lana, Kadohata, and Leybenzon were introduced with the intention that they would be continuing members of the crew, but events in subsequent novels led to their characters going in different directions. Similarly, I created Orilly Malar with the intent that she'd be an ongoing member of Titan's crew, but then I decided that her story arc was best resolved by having her leave the ship. I left the option open for her to return, but subsequent novels moved the ship much farther forward in space and time than I'd expected, so there was no opportunity for her to return. Conversely, I expected Torvig to be merely a supporting character who might only be used on a recurring basis, and I never anticipated that subsequent authors would latch onto him the way they have.
So there is no such category as "regular characters" here. We don't have to worry about actor contracts or availability. We create characters, and the paths they follow are shaped by the evolution of the stories.