Short answers, pulled from the story.
The word character originates from the Ancient Greek term charaktēr, which referred to the stamp or mark impressed upon a coin, signifying ownership and identity. By the time of the Restoration in the 17th century, this term had evolved to describe the chief person in a tragedy. The transition from physical masks to the psychological illusion of being a human person marked a fundamental change in how audiences understood the people on stage.
The concept of character became widely popular only after the publication of Henry Fielding's Tom Jones in 1749. This work cemented the term in the English language as a way to describe a part played by an actor or a person portrayed in a novel. The phrase in character, used to describe an effective impersonation, did not enter common usage until the end of the 18th century.
E. M. Forster defined flat characters as two-dimensional and relatively uncomplicated figures that do not undergo significant development. In contrast, round characters are complex figures with many different characteristics that undergo development, sometimes sufficiently to surprise the reader. Mary Sues are characters that usually appear in fan fiction which are virtually devoid of flaws and are therefore considered flat characters.
Dynamic characters are those that change over the course of the story, while static characters remain the same throughout the narrative. An example of a popular dynamic character in literature is Ebenezer Scrooge, the protagonist of A Christmas Carol by Charles Dickens. At the start of the story, he is a bitter miser, but by the end of the tale, he transforms into a kindhearted, generous man.
In the earliest surviving work of dramatic theory, Poetics, the Classical Greek philosopher Aristotle states that character, or ethos, is one of six qualitative parts of Athenian tragedy. He defines character not to denote a fictional person, but the quality of the person acting in the story and reacting to its situations. Aristotle argues for the primacy of plot, or mythos, over character, stating that without action, a tragedy cannot exist, but without characters, it may.