Short answers, pulled from the story.
The English word character descends from the Ancient Greek χαρακτήρ and came into use during the Restoration period. It became widely used after its appearance in Tom Jones by Henry Fielding in 1749, from which the sense of a part played by an actor developed.
E. M. Forster defined these terms in his book Aspects of the Novel. Flat characters are two-dimensional and relatively uncomplicated, while round characters are complex figures with many different characteristics who are capable of surprising the reader and undergoing development.
In the Poetics, written around 335 BCE, Aristotle argued for the primacy of plot (mythos) over character (ethos). He defined character as that which reveals decision, and held that tragedy represents action and life rather than human beings as such.
A charactonym is a character name that implies the person's psychological makeup, makes an allegorical allusion, or references their appearance. Examples include Shakespeare's Mercutio, Steinbeck's Candy in Of Mice and Men, and Rabelais's giant Gargantua.
A breakout character is a guest or minor character who gains unanticipated popularity and is subsequently promoted to a regular or main role. Unlike core characters, guest characters are not required to be carefully incorporated into the ongoing storyline.
Plautus wrote two centuries after Aristotle, by which time the use of characters to define dramatic genres was well established. His Amphitryon opens with Mercury arguing that because the play contains kings and gods, it must be classified as a tragicomedy rather than a comedy.