If you could take a single character out of a piece of media (book, film, TV show, video game, etc) who would it be?
They would lose any magical powers they may have had in the book, but anything they are, rather than can do, will stay. For example people from the His Dark Materials world would keep their daemons. You can take them out at any time in the story's plot, but for all other people consuming the media, it will be shown that the character suddenly disappears, with the rest of the plot being affected accordingly. People will notice this happening. The character is not under any sort of control by you once you have taken them out of the story, although they will appear next to you to start with.
JFK from right before he was shot. Timed so that the shot happens anyways. Then just let him hang out and absorb the last 60 years for a bit. Then cease to exist or something because him disappearing in the middle of a parade on live television would change history pretty drastically, likely causing me (and most others younger than that) to never be born.