Dictionary.com
Thesaurus.com

flat character vs. round character

flat character
[flat kar-ik-ter] / ˈflæt ˈkær ɪk tər /

noun

  1. an easily recognized character type in fiction who may not be fully delineated but is useful in carrying out some narrative purpose of the author.

round character
[raund kar-ik-ter] / ˈraʊnd ˌkær ɪk tər /

noun

  1. a character in fiction whose personality, background, motives, and other features are fully delineated by the author.