French "person" (side character)

He himself still pronounces his first name with the French ‘j’ sound, but quickly gave up on correcting other people and sometimes himself pronounces it as Jack to avoid confusion or comments. His surname is pronounced "deu-VEES" (rhymes with fleece. There is no English equivalent to the French ‘eu’, so I encourage you to follow your heart on the pronunciation of that). He has not given up on correcting people on that.
Quill Cardenas usually calls him Dr. Device, somewhat sarcastically.

Male (cisgender, he/him, but likely wouldn't correct you if you used other pronouns)

Well over a hundred years old, but looks to be in his late forties or fifties.

Achillean (men loving men) and somewhere on the asexual spectrum

He looks human enough at a passing glance, but anything more than that and it quickly becomes apparent that he’s just not quite of the living. He's tall and spindly to the point of looking distorted; his eyes have dark shadows underneath them, though it's impossible to tell whether this is from fatigue or from the heavy black eye makeup he wears smudging.
Other than looking as though he was put through a mildly fucked up funhouse mirror, he is the epitome of Victorian beauty standards: very thin, very pale, and regularly coughing up alarming amounts of blood.

Rather severely underweight

Well over six feet

Light blond

It's short, about to his collar (though just barely long enough to tie into a ponytail), and flips out a bit at the ends.

Pale blue

Human, at least originally

Deathly pale. His blush shows up as a golden sort of color, not pink or red.

White; French

Tall, thin and spindly to an alarming degree, with long legs and narrow hips and shoulders.

Suit jackets with coattails, vests, button-up shirts, dress pants, long gothic dresses, etc., usually in either all black or all white

Avoids calling people by their actual names, using pet names or something like “kid” instead.
His speech is a little old-fashioned, though not so much that it's distracting. Whenever he thinks "modern," his brain tends to land somewhere in the 1930s. He also purposely uses a large vocabulary and more "proper" language in English as a sort of defense, to prove to himself and others that he's assimilated and can speak English as well as any native speaker can.
Often slips into French whenever he’s talking to himself.
Uses direct translations of French idioms and sayings that aren't real sayings in English, which sometimes makes his metaphors nonsensical or strangely morbid to English speakers.
When he's bored, impatient, or just waiting on something, he'll inspect his nails and/or tap his foot.
He'll often hum or even sing absentmindedly under his breath.
He is nearly always smiling. How is his mouth not tired?

He’s very much a character motivated by curiosity, a desire to understand as much as he possibly can. He’s self-destructive in the sense that he would put his own life on the line without hesitation if he thought it might satisfy his curiosity. If it weren’t for the people he loves, he’d let the world end just to see what would happen next.

To put it simply, Jacques is a coward. He will do anything to protect himself when he feels threatened or scared, and it doesn't take a whole lot to scare him.
He tries very hard to not get attached to new people and often emotionally distances himself from others. He calls almost everyone by pet names or at least only by their surnames so he doesn’t get attached to them. Same idea as scientists not naming animals they’re testing.
Can become obsessive to a frightening degree, especially when it comes to scientific pursuits. Think Victor Frankenstein levels of mania.
It takes a lot to get him mad, but he never lets his anger out in healthy ways, so it just slowly builds in him until he snaps.
Full of Existential Dread™ (not exactly a flaw, but it's gotta go somewhere)
Still bizarrely patriotic about France (this one is a flaw and you cannot tell me otherwise) (désolé, mes amis français)

He's prejudiced towards humans. Despite once being human himself, he generally views them as weak, cowardly and easily manipulated (projecting much?).
Also, centrists.

Serving the Void, an eldritch entity, has made him very powerful in a way that's a bit difficult to explain. It's hard to lock his abilities down into a specific theme or umbrella; they often follow dream-logic associations to the Void and empty space and don't really make logical sense. I'm also being vague because I don't want to spoil anything! The extent of his powers isn't well-known; even he's unsure.
On a less grand scale, he's very good as chess and other strategy games (at least compared to your average mortal. He always loses to Emmett Euclid), and is a very skilled pianist.
His background as a Victorian doctor may, very understandably, seem dubious, but he really does know a lot about the human body and medicine.

To be as vague as humanly possible, science.
Playing chess and other strategy games, usually with Emmett Euclid.
Sometimes he'll play video games with Cyrus Hecate. He can pick up new games very fast, which is infuriating.
He also is very skilled at playing the piano and sometimes sings.
He sometimes makes abstract, often mathematically based drawings--for example, art based off fractals--trying to recreate visually how his mind processes the Void. This usually helps bring him back to the physical world when his mind's wandered a little too far into the eldritch abyss, but sometimes it backfires and he gets obsessed with making the art mathematically perfect. He also makes the occasional anatomical drawing.
As strange as this sounds, operating on himself. When he was first corrupted and realized he couldn’t die by normal means, he would perform little operations on himself just to see how the human body functions. He’s not human, though, so once he figured out all he could in that respect he did...weirder things to himself because he knew he wouldn’t die, and to see how his body now functions. He also has to operate on himself periodically for reasons I’ll get into in the Conditions section.

Device is generally seen as a charming, polite and exact person, if dramatic and off-putting at times.
People will get the feeling they know him relatively well after having several conversations with him, but upon reflection will realize he somehow revealed next to no information about himself.
He's very petty and has weirdly strong opinions on everything, no matter how trivial. A firm believer in the idea that it's much better to be strongly opinionated on stupid things than to be indecisive about everything. Which means he can and will die on the hill that grape is objectively the best candy flavoring.
Competitive as hell.
Generally, when he expresses real anger (which is rarely), his overall manner doesn't seem to change; but he will stiffen a bit, and something about that smile could freeze Hell over.
He pretty much always has a smile on his face as a way to hide his emotions; it's become somewhat automatic. It would be harder for him to not smile at this point. There's not many emotions he outwardly expresses to people besides those he's close with.
Despite this constant smiling, however, he very rarely laughs, even falsely. There's not much he finds genuinely funny, and when he does laugh, he'll startle people who have just been made aware that they’ve never heard him laugh.
On that note, sometimes he'll startle people because he'll laugh at things that are really not supposed to be funny, or at least things that would be considered indecent to laugh at publicly.

Short answer: Jacques is bipolar, which mostly manifests as occasional manic or mixed episodes. In terms of physical conditions, though immortal, he is perpetually dying of tuberculosis. If that wasn't already enough for him to deal with, he's also a puppet to an eldritch horror known as the Void. Though it mostly gives him free reign, he does not technically have free will and it could make him do whatever it wanted if it really felt like it. Fun stuff!

Long-winded explanation: I’m going to try my best to explain as simply as possible the relationship between Jacques and the entity known as the Void here. Buckle up!
First off, Jacques is, for lack of a better phrase, essentially in suspended animation. Towards the end of his mortal life he contracted tuberculosis, likely because as a Victorian doctor he was in contact with so many patients that had it. The Void is keeping him from dropping dead of it (and also just keeping him alive indefinitely), but it in no way cured him, just suspended his body in time. So though he’s immortal, no matter what he does, his tuberculosis cannot improve or worsen. He periodically has to fix his own lungs and replace all the blood he’s coughed up (he makes his own blood substitute these days that he no longer bothers to dye, since all the dyes he's tried just irritate him. Hence why his blush is an odd gold color). So despite having wildly powerful abilities from the Void, physically Jacques is extremely shaky and frail.
The second main point of Jacques’ relationship with the Void is that Jacques does not entirely have free will, and if the Void he serves really wanted to, it could make him do whatever it wanted. He is essentially a meat puppet to the entity and he’s fairly lucky it leaves him alone as much as it does--though it mostly lets him do his thing because he consistently gives it people to consume without question.
Jacques' feelings towards the Void are complicated, to say the least. Because of some things it’s done--like removing all his internal organs besides his lungs and heart or taking a warped version of his own appearance when it needs to talk to him directly--he can’t tell if it has a sick sense of humor or is genuinely trying to be nice to him. And though he sometimes detests what it makes him do, he also in some ways loves and reveres it like someone might love their god.

Towards the end of the story he thinks of Quill Cardenas almost as a little sister.
He's marked as enemies with Dallas Dame for one (1) reason, and that is this: his Victorian, continental French sensibilities are deeply offended by Dally's modern Quebecois French, and at their first meeting Dally used the informal second person pronoun when referring to him. Dally has no idea why Jacques hates him so much, and Jacques has no intention of telling him.
He’s good friends with a cemetery caretaker (aptly known simply as “The Caretaker”) who resides in the Void.

He is very firm on the idea that there are no true gods (despite the fact that there are many entities that could be considered gods), so he’s an atheist.

French is his first language and he did not start learning English until he was in his thirties, but at this point he's fluent and has no noticeable French accent when speaking it.

He doesn’t know enough about the modern political theatre to form an opinion on it. He thinks people should take sides and have strong opinions on political matters, but he doesn't know what any of the sides actually are.

Faithful servant to the physical manifestation of nothingness and valued member of many an interdimensional research project, considering he's able to safely transverse the Void and some other dimensions. He's a big name in some obscure scientific circles and occassionally lectures at colleges about his and other researchers' findings.

Sky blue

He doesn't need to eat and so usually doesn't, but he loves any and all grape-flavored candy.

His wedding ring. It’s black and in the shape of a snake eating its own tail.

He doesn't have a favorite animal, but he really likes trees. That has to count for something, right?

November 21

Trained as a doctor in the late 19th century. He has no formal training beyond that, but he’s made a point of updating his medical knowledge with the times.

TL;DR: Once a doctor living in mid 1800s France and eventually America, Jacques messed with things he shouldn't've and became a puppet to an entity called the Void.

Device started out as a doctor in mid-1800s France. He was relatively forward-thinking for his time, but he did not, in fact, care very much about keeping people alive. Several people accused him of malpractice, but he was never condemned for anything.
After practicing medicine for some time, Device became interested in the newer field of interdimensional science. He quickly proved himself to have the right kind of mind for staring into the eldritch unknown without going completely mad, which was generally the biggest problem faced by the thinkers of the day, and soon made a name for himself among the small but growing community of scientists. Soon enough, he found himself traveling to America to give lectures (not a great idea, considering he knew next to no English at the time) and to talk to an American scientist named Dr. Emmett Euclid who shared a similar talent for peering into the abyss.
Device and Euclid became very close (historians who are not aware they are alive and married in the present day might call them "good friends") during the course of Device's stay, and so Device decided to stay in America permanently to be with him and to further realize their ideas. Though their research—which mostly consisted of finding ways to reliably go back and forth between dimensions—made huge leaps in interdimensional science in general, Device and Euclid are mainly known for their idea of burying the dead in necropoli in the physical manifestation of nothing known as the Void via railway. Necropolis railways took off in America and soon spread across the rest of the world; to this day, you'd be hard-pressed to find a cemetery on Earth with graves dated any later than the early 1900s.
Shortly after this breakthrough, when he was in his early fifties and had been living in America for about a decade, Device contracted tuberculosis. Believing he wasn’t going to live much longer anyway, he became even more self-destructive with his research and borderline suicidal. The Void, however, could not have been more pleased with him: as it would turn out, dumping the dead into the physical embodiment of nothing has about the same effect as dunking a Magic Grow™ dinosaur foam capsule into water. The Void, seeing the doctor didn't have much time left, decided it would help him out because he helped expand it so much. And so, Dr. Jacques Device, despite his vehement aversion to the whole thing, became the Void's faithful servant. Forever!
This, understandably, fucked him up pretty badly. He became bound to the Void, becoming its puppet, ending up immortal and very not human in the process. The Void seems to have a fondness for him, and he's not quite sure if that makes him lucky or not. It generally gives him free reign as long as he willingly helps it expand.
Device never quite got the fame he perhaps deserved for his breakthroughs, though if you work in certain fields of interdimensional science, you definitely know who he is. He's still active in some scientific circles in the modern day. His peers have a pretty good idea as to why he hasn't aged a day in a hundred years, but it seems a sensitive subject and they have enough tact to not bring it up.

Neutral…something, that's for sure. Neutral evil? Neutral evil-ish? Really depends on how you view morality. He's not a "bad" person, but he's a coward who will do bad things when he's scared for himself or the people he loves, and he's scared a lot.
Then again, he also does some really "good" things, like, y'know, trying to stop the apocalypse. But in order to be good, do you really have to be good, or just perform good actions? Is someone good if that goodness only really extends to their loved ones? I leave it up to you!

Will Wood, more specifically the monologue in his song "Blackboxwarrior - OKULTRA"

Here's the Spotify playlist I made for him in its entirety if you're interested! Here's some songs that remind me of him the most:
- Basically the entirety of Will Wood's The Normal Album, but especially "Blackboxwarrior - OKULTRA"
- “Rule #21: Momento Mori” by Fish in a Birdcage
- "Maledetta Orangina" by the Petrojvic Blasting Company
- "Les Yeux Noirs" by Pomplamoose
- "Lucifer Yellow" by the Choir Vandals
- “We Will All Go Together When We Go” by Tom Lehrer

Jack's Pinterest board (content warning for a few gory illustrations and some general creepiness)

He has a refsheet.net profile that's shorter than this one and also way cooler-looking (the power of CSS!) that you should totally check out if you feel like it :-]

All art in the image gallery drawn by me!

Some miscellaneous stuff and fun facts:

He and Emmett Euclid are married!

He’s fluent in French and English, and can speak barely passable Latin.
He's originally from France and still uses the language when talking to himself or his husband, but he hasn’t lived there in a long time and no longer has a recognizably French accent when speaking English (though it becomes more apparent when he says certain words, and he still occasionally forgets the English words for things). As it's been a solid hundred or so years since he’s lived in France, his French, though perfect, is also somewhat formal and old-fashioned. When he first meets Dallas Dame he’s horribly taken aback by his Québécois French and nearly has a stroke after being addressed as tu.

Device is severely claustrophobic and dislikes any enclosed space without a clear view of the outside. He refuses to enter underground spaces. This is likely a side effect of the Void, which loves openness and empty space; however, it's worth noting that he was claustrophobic to some extent before encountering it.