Hi saitoki, the one character is only for the "sprite", which means you can have many more characters in your story, also interacting, as long as they are not shown. For example you can have a press conference where a person (1 sprite) is in a press area (1 background), and you can have several reporters ask questions.
The same goes with the second question, you can mention and interact with as many characters as you need to (in the present or past), as long as only one sprite is used in the game.