When I think about character designs for my own characters, I pick the things like their role in the story, I name a few of their key ‘issues’ and struggles, I decide how attached I want my readers to be with this character, and then I begin curating a long list of headcanons. (Sometimes I have a death scene in mind, or a name that feels right.)

With this bulleted list of variably niche things my character will do, or would seem within their brand to do, I have an idea of what vibe, what kind of energy they will give off. From there, I begin crafting their personality, and I usually start with the dialogue of how they interact with other characters.

If we’re just flinging ideas out here, here’s a couple of my takes on Jack.

Jack is the overlooked social butterfly, in the way that he wasn’t necessarily social, but so many people from different corners of a campus wanted to be friends with him that he just drifted from one lunch spot to another and listened to people talking to him. He’s not the talkative kind, but he’s quick-witted around other people with sharp tongues. He played one sport for a while, but quit when his friend did. Since then he’s let his hair grow out a bit so the little kids at the local preschool can put butterfly clips and make little pigtails. He lets them practice their hairstyles on him. He gets headaches from overly sweet fragrances like vanilla air fresheners and cheap citrus candles, but he loves lavender and musky colognes. He'll readily pretend to be someone's boyfriend in a pinch, but flirting with him under that excuse is so easy to read. He messes with teachers if he knows they’ll blame someone else for his nonsense, and he would wear eyeliner to places no one will recognize him. He knows he looks good, but it’s too much to deal with for anyone more than a stranger. He can come off as distant and passive, but he's really just lived too long to get attached to people that easily. Half the time he's aloof and absorbing everything around him, but I get the feeling he's not lost in thought. It's almost like he sees things he knows he shouldn't.

Alternatively:

He simultaneously has a god complex and an inferiority complex. His cockiness and confidence sometimes rubs people the wrong way, but those who put up with him and show their affection in unconventional ways know he embodies ‘fake it ‘til you make it.’ (His closest friend is always mean to him). He’s charming the way a dog is handsome after a haircut; a little confused, but he knows something’s special about him. His hair is stripped from pool chlorine or sea salt, yet it still shines like silk when he gives a damn and a half. There’s this clunky ring he prizes and wears on a necklace sometimes, but he’s never put it on his finger. He has his reasons, but he says the gold metal clashes with his skin’s undertones no matter how he styles it. Gold? People believe him when he grins like that, of course. When he fails, he fixates on it and drills himself half to death until he can show how much he’s learned from it. That's what makes him good at sports and pronouns. Sometimes when he's lost in repetition, you'll hear him muttering some indecipherable, foreign language like he's in a trance. Call out to him though and he'll reset to his usual prince self in a blink.

Also Alternatively:

He read "Stargirl" for an assignment once and used it as the basis for his aesthetic for almost a decade. He did weird shit like brew coffee with sparkling water, color in strands of his hair with sharpie every other week, and tape his paper together instead of using a stapler. At some point, when he moved from one school to another he completely changed his style and taste. Maybe he found a new book he liked. His actions screamed 'don't look at me' but his trendsetter style contradicted that. It almost looked like he was perpetually on a runway with the way he would rather be caught dead than leaving the house unphotogenic. It was effortless yet intentional, random yet measured, and editorial but candid. When he spoke, it was all quotable. If it wasn't caption worthy, it was ASMR material for sure. Despite that radical change, he still acted the same, for the most part. It was quirky now because he was like a model, but the fucker probably cleans pennies and turns bar soap into liquid soap on weekends still. Some cool kid he is. Rumor has it he has this crazy tattoo on him. Some people say it's huge, others said it was small dainty. No one can agree on where it is on him, not even the boys who used to see him in the locker room. Most people agree it's black and white, but I could've sworn I'd seen some color on it. He must have multiple tattoos then, since the one I've seen was on the back of his head where his hair hides it when it's grown a bit. He insists there's only one, but who really knows? Maybe it moves and changes, haha. maybe.

If you’re thinking of writing a romantic mystical story, then the desriptions you have are on the right track... but I might tweak them just a little bit, just bear with me.

Okay, so I might switch the hair color a bit... put in some darker brown highlights. Jack will have Electric blue eyes with hints of violet and deep green. You didn’t give an age range so I’ll say he’s around 19 in looks, but is 500 years old due to his vampire side. Jack is also in a school for magically tallented beings, and is learning dark magic, healing magic, and protective magic. He is also learning good magic. His hobbies will include astronomy, music, sports, reading, writing, etc. Jack will probably be a celebrity in his home world. Though he is 45% vampire he does not drink blood, he does however eat meat (not human meat). His possible motivation and drive could be a girl from his school who is 45% werewolf, and is the same age as him. Jack also has perfectly straight, pearly white teeth.

Thoughts:

Jack does sound ruggedly attractive, if one is to go off of his description. Most boys with this description end up being Don Juan’s and Casanova’s (in my opinion). Extroverted bad boys (or seemingly “bad” boys) who chase after girls.

How about if we could make him a little more “original”?

Ideas:

INFJ-A - Introverted, Intuitive, Feeling, Judging, Assertive

How about Jack is an introvert? He is an intuitive human being, which means that he lives mostly inside his head. Instead of being a thinker, he is a feeler (heart over head). Being judging means that he wants to get things done the way they should be done. As for assertive: It just means that he stays calm when a storm breaks out in his life. He can handle it pretty well. He is confident.

Potential Hobbies: Surfing, Hiking, Deep-sea diving

Jack has an athletic build; therefore, I assume that he likes to keep himself busy with active endeavours. Surfing might be an obvious choice. Deep-sea diving, too. He could have all three hobbies, and make time for each on different days during a week.

(I include this, since I feel hobbies tell one a lot about a character)

Drive: Hmm...

Give Jack a dream. Something that he can believe in passionately, and that he will leave everything for in order to pursue it.

"[INFJs] have a deep sense of idealism and integrity, but they aren’t idle dreamers – they take concrete steps to realize their goals and make a lasting impact."

SOURCES: (Since I'm not naturally this knowledgeable on INFJs.)

https://www.16personalities.com/infj-personality

https://www.16personalities.com/articles/assertive-advocate-infj-a-vs-turbulent-advocate-infj-t

All depends really, but here goes-

Swordsman?

Really good cook.

Sometimes puts his hair back with headband?

Actually fairly smart, just doesn’t show it.

Likes competition.

Really good with kids, patient.

If he feels threatened by someone older, he gets really mad.

Likes running.

Dense as a brick wall when it comes to romance.

Can't read people well, but gets better at it over time.

Really bad at reading maps.

Now for drive-

Needs to find a certain ingredient to bring back so his youngest sister can finish the excorsism on the princess. He loves his siblings so he will do anything for them. He takes the second oldest sister along on his quest to find the ingredient because she begged so much to go with him. And he wants to prove himself as a good swordsman to his older sister on the side for some reason.