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My personality analysis of the lover of Lily Evans, the victim of the Marauders, the saver of the wizarding world, the object of my affection, and "the bravest man" Harry Potter ever knew:
Look how agreeable he looks!
Openness: At first glance slash meeting, the average human being would not describe Severus Snape as particularly "Open to New Experiences." However, by the end of The Half-Blood Prince, we readers have realized that Snape is an intellectually curious, inventive, and powerful wizard/. After all, a Closed wizard could hardly have attained a vast enough knowledge of the Dark Arts to win a duel against three Hogwarts professors; save Albus Dumbledore, Katie Bell, Draco Malfoy, and countless others' lives; and successfully pull the wool over Lord Voldemort's eyes.
Conscientiousness: If Severus Snape is anything, he is careful (aside from situations involving Harry, Sirius, or a challenge of cowardice - in those instances, reckless might be a better adjective). Only an intensely vigilant, calculating, and conscientious individual could have successfully earned and kept the trust of both the goodest and the evilest wizards in the wizarding world (AND kept his true loyalties a secret) until the very end.
Extraversion: I'm sure by now that you've sensed a very defensive pattern in this analysis. That ends here. Snape was one introverted dude.
Agreeableness: "Sociable and easy to get along with." ...BAHAHAHA. Snape, easy to get along with. That's a good one.
Neuroticism: I lied. If Severus Snape is anything, he is neurotic... though perhaps only on the outside. It's true that he's eternally depicted as tense, moody, and socially maladjusted, but there are only a handful of times in the series where Snape is beset by feelings of anxiety, compulsiveness, or obsessiveness. He would view such feelings as weaknesses and refuse to be hindered by them.