A catalyst character, also known as a catalytic character, is the character in a story who causes the protagonist, or main character, to move toward some kind of action or transformation. This character is usually the person that spends the most quality and influential time with the protagonist.
The catalyst character can sometimes be mistaken for the protagonist, as the catalyst character is present throughout all or much of the story. Writers use catalyst characters to draw the protagonist's attention to a specific change that needs to happen while ensuring the character does not outshine the protagonist once that change is set in motion.
Some popular catalyst characters are Jack from "Titanic," Obi-Wan Kenobi from "Star Wars" and Mercutio from William Shakespeare's "Romeo and Juliet." In "Titanic," Jack makes Rose realize that she cannot continue to live the life of blind privilege she has unhappily been living. By the time Jack dies, she knows she has to start over completely, so she hides from her family and makes her own way. In "Star Wars," Kenobi influences Luke Skywalker to take action when he learns the Empire is responsible for killing his aunt and uncle. In "Romeo and Juliet," Mercutio influences Romeo's relationship with Juliet while his death moves the action toward the tragic ending of the play.