⚶ Personality ⚶
"Socially awkward" would be an understatement. For the first ten years of his life, he was rarely given anything resembling affection, as many of the researchers who raised him feared growing close to him emotionally, for fear that he would "melt down" -- figuratively or literally!
This has made him seem like a rather cold individual to the average person, though he remains polite as much as possible. "Coolly polite" might be a good way for an average observer to describe him. This reticence isn't out of rudeness; he just doesn't like to burden people he feels he may never see again with his problems. Even those who run in the same circles he does -- adventurers mostly -- generally don't get a lot out of him. He's never really had a "friend" (or anything else), so he doesn't really know how to unburden himself in this way.
Besides that, he's not sure the inquisitors of Ishgard have given up pursuing him, even after the end of the Dragonsong War. Anger tends to smolder for much longer than the act that gave rise to it, he knows. He isn't altogether certain that someone isn't still looking for him, and the more information that's floating around in a pool of people, the more information someone who may be pursuing him has access to...
His upbringing has made it difficult for him to connect with people in the easy sort of way that many others do. However, while being raised by scientific people who kept him at a distance has made him a logical sort, his years in Ishgard taught him that emotions are very rarely logical. Yet he knows that they are no less real or valid to those who experience them. His own emotions can overcome him at times, when they are very strong or set upon him very suddenly, since he never learned how to dampen or hide them when they grew very strong. To the Sharlayan researchers, these were "fits" to be observed carefully, not merely the completely normal high-running emotions of a child who needed comfort.
Courtois doesn't talk about his upbringing or his origins casually, as he dislikes the barriers they near-instantly place between himself and the people around him. He doesn't want to be special. He wants to be a normal person for once, even if he's just "that weird guy who sits there and people-watches; he doesn't say much but he doesn't bother anybody". When asked about his origins he'll generally just say he's from Ishgard (often adding "unfortunately" somewhere in the sentence), but that he wasn't exactly "popular" there. It's just something else that he doesn't like to talk about with the average person.
He doesn't actually hate Ishgard or her people for the circumstances of his leaving. He does hold a grudge against the inquisitor who actually sentenced him, of course -- because he knows the sentence wasn't out of a genuine belief that he was a danger to the people of Ishgard, but out of a selfish urge to remove a mouth that was speaking words said inquisitor didn't like.
His scientific upbringing has made him a curious sort however, and he tends to approach problems—his own as well as others’—with a logical mind, to create a theory of how to “solve” them. This isn’t always possible, though. And he is generally happy to leave a problem be if he’s asked. That’s one of the ways that one might guess that he’s developing a fondness for them, in fact—if the idea of leaving unsolved a problem an associate has bothers him.
Courtois has no idea what he would like in a romantic partner. In fact, he has no idea that he would want a romantic partner at all. As merely a ward of the minor Ishgardian noble house that took him in, he was not expected to marry into a family “of his station”, and there was nothing for any other house to gain politically by establishing any connections with him, friendship or otherwise.
For those considering Courtois for romantic pursuit? Don’t. He spent the first ten years of his life as someone’s science project, then three years on the run, then seven years as an outsider in Ishgard. He has nothing to offer a prospective romantic partner, and would require too much educating to be worth it for the typical person. This is especially true of those who would make an effort to “bag” Courtois as a one night stand. He does not have the “romantic maturity” to properly handle amicably parting ways with someone with whom he was intimate.