I’m writing a story and I want to use already existing characters because of the scenario. Can I do this?

If the person is a well-known person who is often in the news, then, yes, you can use them if what you say is absolutely, positively true with no possibility of any misunderstanding and it’s difficult to know where the line is between what’s allowed and what’s not.

What I advise for any and all such situations is that you use another name for those people. This can work as long as you insist that your character is fictional and you make enough small changes to their characteristics that, though everyone knows whom you are referencing, your character is different enough to truly be considered fictional.

In other words, remember what I said earlier. Be very, very careful and, if at all possible, disguise that person as a fictional character.