How are new regular characters developed for the relaunches? Does an author pitch ideas for new regulars to the editor? Or does the editor say, "Here's the sort of character we'll need. See what you can do with this outline"? Or does everyone get together and brainstorm?
^ A little bit of both. Though the Star Trek Vanguard series was not a "relaunch," its cast of original characters was a mix of those proposed by editor Marco Palmieri and myself.
The characters I've created who've become regular or recurring were just ones I came up with on my own. For Torvig, Chaka, and Orilly from Titan, I wrote up descriptions that I sent to Marco and the other authors as sort of a supplement to the series bible; for the others, I just described them in the outline, like T'Ryssa Chen, or just put them in the manuscript, like Rennan Konya in SCE. In every case, of course, it has to be approved by the editor, along with everything else.
When you created Trys, did you create her as a new regular character, or was she accepted as one after started and/or finished the book?
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.
Christopher, I wouldn't totally agree with you that 'regular characters' don't exist in novels, although you're right that the term 'regular character' is a bit of a misnomer when you're dealing with literary works. 'Main Character' would be more appropriate, although the distinction isn't quite as 'set-in-stone' as its television and movie correlation(s), since sometimes the development of a literary story will lead to 'secondary characters' becoming 'main characters' when they weren't originally intended to be. For example, I'm currently working on a piece of fanfiction featuring Albus Severus Potter, Harry's youngest son, and his classmates and contemporaries at Hogwarts, and one of the original characters I've created has taken on a much bigger role through the first part of the story than I was originally intending, turning what was originally going to be a 'trio' of 'main characters' into a quartet.