• The journal name is kind of a misnomer! Jack is more of a sociopath than a psychopath; he is controlled and calculating and he's spent most of a decade century being a social predator.
• Jack is a manipulative chessmaster and the only reason he is befriending you is because you are either a) useful or b) interestingly useful or c) amusing useful. Or a combination of the three. Or he probably knows you from his canon.
However, he is not impervious to loneliness, and persistence, so if you can actually see past his assholery, Jack makes for a...slightly less terrifying person. Slightly.
• he is obsessed with the Baskerville siblings - Lacie, who has taught him how to live, and Oswald, who has taught him that he doesn't want to.
As long as there is an Oswald or a Lacie in-game, he will follow whatever they say.
• He is usually a happy derp who loves flirting with the ladies and climbing trees. Trees, his first love, he might be part squirrel with the amount of time he spends up in the branches. However, when he is alone, or when he has gotten tired of someone, he is a murderous psychopath who has zero regret when throwing people away. He isn't always all there.
• He is also quite skilled with his hands, having watchmaking and jewelry as his hobby. He can play stringed instruments, and he can also sing. He doesn't like singing, though. He's also blessed with stunning good looks and charisma.
• Jack cannot age past 24, he just rewinds into a child and ages up until he's 24 again, soul disintegrating with each cycle, repeating until there is none of his soul left in his body.
SOMARIUM + EXSILIUM
take this with a grain of salt, since there's a lot of headcanon here
• thankfully Jack is slowly learning empathy though, by virtue of Oswald being his guinea pig, but don't expect him to live up to his created good and slightly airheaded nobleman persona. He can be a hell of a brat, since he'd never emotionally matured well, having shut himself down in pursuit of Lacie. He whines a lot and has zero regard for personal space, and to an extent, people's feelings. He also likes to tease and stare at people, making them flustered and/or angry, because he likes the feeling of inciting reactions from other people.
• he will follow anything Oswald says, so the best way to get him to do or stop doing something is ask the other man to intercede. Also, the best way to piss Jack off is to do something detrimental to Oswald in any way, shape or form.
• Jack is vain. He did have to keep up an impeccable appearance for the harsh judgment of the upper echelons of society, and he'd kept it up for nearly a century, so keep grubby hands away from his hair. He is also kind of touchy about his hair; he grew it out in imitation of Lacie's, so don't diss his braid.
(He is also horrified by Crocs. Get them away.)
• He has the talent to take a nap anywhere, anytime, in less than a minute.
• Jack doesn't really have a sense of modesty, he just goes along with social cues and such. He likes to flirt, but honestly, nothing will come out of it, since Oswald already gave him the silent treatment about it... maybe a friendly date. Wherein Jack promises no maiming or torturing after dinner.
• While all noblemen were expected to learn the art of the sword, Jack learnt it too, and he can hold his own against a swordmaster. He also fights dirty. Very dirtily, if push comes to shove, but he won't do this unless he wants to kill his opponent outright. (Which...is often...orz.)
Jack is also well-versed in some magic spells, Abyss lore, and poisons. He is very, very well-spoken if he wishes to be, and has fooled an entire country for a hundred years. His charisma, ability to make people believe anything, combined with his ruthlessness and lack of empathy makes him a very dangerous man should you get in his way. He can and will still kill with no remorse; the only reason why he doesn't do so is because Oswald would frown on him doing it.