Final answer:
Fully developed characters in literature with distinct personalities and background are called round characters, as opposed to flat characters who are more one-dimensional. Round characters are complex and usually undergo development, while foils serve to highlight contrasting traits in other characters.
Step-by-step explanation:
Fully developed characters that have a distinct personality, background, and motives in a story or novel are known as round characters. These characters are contrasted with flat characters, who are more one-dimensional and are often based on stereotypes. E.M. Forster introduced the concept of flat and round characters in his book, Aspects of the Novel, where he described flat characters as simplistic, while round characters are complex and undergo development throughout the story. Another type of character is the foil, whose traits contrast with those of the main character, thus highlighting particular aspects of that character. For example, a decisive and determined foil can draw attention to a protagonist's indecisiveness by contrast.