I would like to create a few characters for npcs and big baddies, but I I noticed by logging in with a throwaway account that I could still see the list of characters created for that campaign, even those I had deactivated with my primary account. Those character sheets are set to private so I could not see them fully, but I could still read the names, race, class(es)... which would ruin the surprise once I employed them in the game.

So.. can I create characters and NPCs and have them listed in the campaign but not visible to my players?

Make a separate campaign that doesn't have players, move characters to there.

are you telling me I can move characters in and out of a campaign? I knew I could have a character leave one.. didn't know having it join one was an option

If the character is assigned to you and you remove it from campaign it becomes an ordinary character you own, you can then make a new campaign, use that campaign's join link and select your character to add it there - repeat for each character you want to move.

Using campaigns this way is a great way to organise NPCs. There is no limit to how many campaigns you have, or how many characters it can have.

of course...! the join link!
thanks for the tip.. I hadn't thought of that.

Yep.  This is what I do.  Works like a charm.  And since you own it you don't have to be worried about it counting toward your share limit or anything else.  Whether it is NPCs that help the party or Villains that work against the party... typically I don't develop too many but, I guess if I ever get too many in one place and it blocks me, then I'll subdivide those into further groups.  Now if we just had a way to rearrange the Campaigns by name instead of Date Created.