A little background information: Since we're all busy adults, our group plays mostly via chat instead of meeting in-person. And I don't mean voice chat, but taking turns typing out the actions of your characters in an actual text-based chat. Occasionally, we get a newbie wanting to join and usually embrace them with open arms.
Recently a new player joined our group and gameplay has been a hot mess since then, as she legit can't distinguish between "her stories & NPCs" and the ones that belong to other people. I've been playing for more than 20 years and literally never had this problem with anyone, as I always thought it was an unspoken rule amongst roleplayers that characters and storylines belong to the people who introduced them.
One recent example: I was trying to DM a simple story - haunted house, desparate questgiver trying to drive the ghost out, nothing too complicated but I've had prepared the whole story nonetheless and slowly introduced more information to the group. Then they meet the actual questgiver, and suddenly SHE takes control of the story, describes in detail what the questgiver said about the house and what happened there, introduces his wife and kids (her creations, not mine) and then graciously allowed the others to continue after basically hijacking my plot. She did not ask beforehand whether it was ok, nor did she know anything about the plot that I wanted to DM.
We were all flabberghasted and tried to explain to her that she can introduce her own NPCs whenever she wants but should leave the effing PLOT to the DM. She didn't understand what she did wrong and started asking questions about which characters she was "allowed" to play, then seemed confused when the obvious answer was always; only your own.
I have nothing against my players temporarily taking control of unimportant "faceless" standard NPCs like stable boys, waitresses in a tavern etc. to speed up the unimportant parts of the gameplay like ordering food and the like, but those were named, detailed NPCs that were very obviously important to the overarching story and originally introduced by me. And the best part is that she then expected the other players to take control of the wife and kid SHE tossed into the game, because her character started interacting with them and she didn't want to "play by herself".
Similar situations have happened with the NPCs of other players. She takes control of NPCs that are not her own, expects other people to take control of NPCs she introduced herself, and doesn't get why we won't do that. How are other people supposed to know the background stories of HER NPCs?!
I have never had such a player in over two decades yet she claims that this is how she had always played and it had never been an issue for anyone. And I currently have no idea how to deal with it. On one hand, she is a friendly person and her characters fit right into the universe we created, but on the other hand it drives me nuts that I have to get my mind out of the in-game zone every three paragraphs to tell her to stop doing this stuff. If this continues I will have to kick her out of the group, which I'd like to avoid .. but I can't seem to make her understand the concept of "leave the creations of other people to them".
Did anyone have similar experiences with a player? And if so, how did you deal with it?
Uh-oh, you seem to have a storygamer on your hands. Some newer systems such as PBTA and Forged in Darkness have a "pass the GM stick around" mechanism, and borders between players and GM are slightly more fluid. I presume this is her first time playing a traditional RPG, so I would recommend taking her aside and telling her that in D&D (which I assume you're playing) players play only their characters, while the DM plays the rest of the world.
This is not the way PbtA or FitD games work. You can make your point without trying to poorly rag on games other people enjoy.
There are plenty of "story games" out there that do pass the stick, but neither of the games you mentioned are of that type.