Disclaimer: This is not one of my most authoritative posts. The initial idea came from my early grad school notes, which is why the big ideas are marked as coming from my professors and not from books. I may not be of great use in answering questions, but if you have some, send them in and I’ll do my best.
One of the most interesting ideas I learned from my first graduate medieval lit class is that psychology and personality were never driving forces for literary characters in the Middle Ages. The concepts of “psychology” and “personality” did not exist in the Middle Ages and really did not exist until the 18th C. Nothing was thought of in political  or psychological terms—everything was identified in terms of what’s best for the community and what’s moral, right or wrong, good or evil. The term “personality” as we use it today does not enter the English language until the 18th C, which my professors emphasized was an idea of an industrial society in which humans find themselves isolated despite being in large groups.
The OED lists “personality” as meaning “The quality or collection of qualities which makes a person a distinctive individual,” first appearing in 1710. As a psychological term, it first appears in 1930. The word “personality” does appear in the English language as early as 1425, but this definition refers to qualities that make a human distinct from an animal, thing, or idea. 
A character in medieval literature is highly unlikely to be driven by interiority. If a character is evil-natured, it is not the result of psychological trauma or social problems. He simply is. Mordred does not develop into a cruel character over the course of an Arthurian tale—he is born that way. I’ve yet to find a medieval work that gives Mordred a developed descent into evil the way that so many modern works do.
This does not mean a character is never motivated by what happens to him. Cruel habits can be learned. Just as being raised by good parents credits a child with goodness, so being raised by bad parents can credit a child with badness. However, take for comparison the portrayal of Sir Gawaine in T.H. White’s The Once and Future King versus Gawain in Sir Gawain and the Green Knight. The poor qualities of T.H. White’s Gawaine are the result of a mother who taught him to adore her completely, but gave him nothing in return. This resulted in him developing a mistrust and even fear of outsiders and an insecurity of abandonment from growing up impoverished. These ideas would have been foreign to medieval literature. Sir Gawain of SGGK is pure by nature, and though he learns a lesson about the risks appropriate to knights and the value of truth-telling, his nature remains unchanged.
In medieval literature, inherent goodness is always noticed. In Chrétien de Troyes’s Perceval, though ragged, uneducated Perceval blunders his way into Arthur’s hall, Arthur rebukes Kay for treating Perceval harshly: “You’re very wrong to mock this lad: that’s a very grave fault in a gentleman. Although the youth is naive, he may well be of good birth; for it’s a matter of upbringing, and he has learnt under a bad master.”  Soon, Perceval is on the same footing as all the other knights.
In Malory’s Le Morte Darthur, Gareth makes a similar appearance. Gareth arrives with several other men, so he doesn’t come in looking like the lowest of peasants, but nobody knows who he is. Gareth asks Arthur for only food and drink while he stays, but Arthur begs him to ask for greater gifts because “this is but a simple askyng; for myne herte gyvyth me to the gretly, that thou arte com of men of worshyp—and gretly my conceyte fayleth me but thou shalt preve a man of ryght grete worshyp.”  Gawain, Lancelot, and Arthur all recognize Gareth’s inherent nobility without knowing who he is. Kay does not recognize it, but Kay is a jerk.
The idea of inherent nobility being recognized despite a character’s circumstances is continued even through Victorian literature. The Victorian novel is filled with characters descended from the aristocracy who are born into undesirable circumstances, yet speak with high-class diction. These characters always find their stations by the end of the novel. Understanding the medieval (lack of) definition of personality can help new readers understand why many medieval characters seem a bit stereotyped and what medieval readers expected from their fiction.
- I expect there will be a great deal of contention over this one. Remember that absolute monarchy did not exist, affiliation to one’s country was of secondary importance to affiliation to God, and the state was a loose, poorly-defined thing. A medieval political concern is usually best understood as relating to a large community.
- Oxford English Dictionary, 2nd ed., s.v. “personality.”
- De Troyes, Chrétien, “Perceval,” Arthurian Romances, trans. D.D.R. Owen, (London, Everyman: 1991), 387.
- Malory, Thomas, “Sir Gareth of Orkeney,” Le Morte Darthur, ed. Stephen H.A. Shepherd, (New York: W.W. Norton & Company, 2004), 178.