A supporting character is a character in a narrative that is not focused on by the primary storyline. Sometimes supporting characters may develop a complex back-story of their own, but this is usually in relation to the main character, rather than entirely independently. Supporting characters may appear in more than half of the episodes per season.
In some cases, especially in ongoing material such as comic books and television series, supporting characters themselves may become main characters in a spin-off if they are sufficiently popular with fans.
See also