Foil
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Character Role Analysis
Raymond Babbitt, Charlie Babbitt
Raymond is the foil because he and his qualities drive changes to Charlie's life and attitudes. Charlie ends up growing into a more successful, stable, and (definitely) nicer individual after having to deal with all of the ways he and Raymond are different—and, more to the point, having to learn how to accommodate someone who is different.
Think about it— Charlie cut himself from his dad because they disagreed about whether Charlie should be driving the family car, which tells you he really values having his way.
So, it's no surprise that having to adjust basically everything—including eating, sleeping, and traveling habits—to accommodate Ray's special needs was a big adjustment. By dealing with all of Ray's differences and what not, Charlie learned how to be empathetic and kind, which seems like nothing short of a miracle, given where he started out. Ray was the perfect foil to him.