I am on PC but I think PS5 might work the same.
If you start a new campaign and you want other people to join your game, they all need to make a custom character. Only the host can select an Origin character. So when your girlfriend made her custom character she will join your game. She then can take along every NPC in your camp, but cannot talk to the NPC who is attached to you. Just save the game after she joined, and next time she can just play with her character already in your game.
When she quits the game, you as host can still load the save and her character will still be in your party. and you could continue the game in singleplayer. If you would like to "park" her custom character, just dismiss her from your group and she will go into a wardrobe in your camp. Should she log in again in the future, she would automatically take control of her original character she made.
Your second example, where the game kicks out a NPC, is always the case as the aforementioned fact that, other than the host, other people cannot play as Origin characters.
Hope this helps!