I had an interesting conversation with Eric Flint at Pensacon last weekend on creating the Dynamic Duo. Victor Cachet is all Eric but he chuckled about having to get to SVW to find an existing Manticorian character as the 'counterpoint' that the Mad Wizard hadn't already killed off! The brief scene in SVW that introduced Anton Zilwicki provides very little on him so besides what ever existed on him in the Honorverse 'bible' at the time, it's FtH that creates the character we know.
A couple of asides on that SVW scene; first Tom Thiesman is the PRN flag captain whose competence barely overcomes the Commodore's incompetence to force Captain Helen Zilwicki's Saganami Moment and secondly, I look forward to finding out the name of her light cruiser, which I'm sure is on the RMN's honor list, when Captain Helen Zilwicki #2 takes command.
FtH also introduces several other characters who become significant starting out with the very next HH book, WoH. While Catherine Montaigne, Jeremy X, Kevin (he is in short SVW scene too) and Virginia Usher were probably in the HH 'bible' already, only Eric and the MWW know in how much detail.