Diamond lives out in the boonies of New Jersey, working by day at a steel mill and by night in their workshop. It's cheap, it's got plenty of space for storing scrap, and they got a deal on the whole thing by agreeing to restore the barn in exchange for paying zero rent. They're just barely not so far out of town that a trip into town is impossible, and their workplace sits on the edge of the town proper, making it a convenient midpoint between home and groceries. They used to be really big into muscle cars, but the scene dried up as the people who could afford those toys moved onto less shitty rural towns to live and drive in.
Diamond works for New Jersey Steel Baron, a steel mill specializing in making blades for the New Jersey area. Specifically, Diamond works in servicing the needed machinery and ensuring the production line keeps working. When on the job, it consumes their entire focus, lending an air of irritability to them as their words become quick and concise. When they're off the job, however, every cent not spent on subsistence is instead promptly invested into materials to complete their goal, resulting in the purchases of textbooks, occultist manifestos, steel, supplies for 3D printers and automated milling machines, and a lot of medical equipment they're trying to teach themselves to understand. They're currently saving up to build a modular cleanroom, able to be removed from the barn (per landlord's request) if they ever moved out, and intended for storing things like backup organs, pre-prepared mechanical replacements, and other things they intend to have forced into their body at some point.
Diamond Jasper wants to end the separation between mind and body - the Orb of Light Transhumanism concept, taken to the extreme. However, they are unsatisfied with answering this question merely by abandoning the physical - they are convinced that the realm of the physical has important and meaningful significance to the mind, and that the suffering inherent to having a body is part of the experience of Being. A lack of a meatsuit just gives you Different Bodily Traumas, and so Diamond seeks to create a body whose traumas are ones they designed in advance. They see every injury of theirs as an opportunity to build themselves up and replace the flawed meat with the perfection of Steel, and to that end study the occult for ways to bind the Mind to the artificial, in the hopes that they can create something truly theirs to inhabit. They hope to be strong like the terminator, with a flawless memory, and retain modularity so as to be able to swap and replace their parts with ease, ensuring they can be upgraded and maintained far into the future.
When Diamond was young, they learned who Hatsune Miku was, an entirely fabricated fictional android who existed only in the art of her music. Prior to then, while they had been aware of entirely fictional characters, a community-made beast like Miku had not existed in their social paradigm before. Upon learning what a vocaloid was, they frantically created their own, even going so far as to create the voicebank for this vocaloid (named Jun/"Talented") out of their own voice. This began their obsession with their current goal, and when they eventually went to technical college, they created a little Arduino to use Jun's voice for them by going speech -> text -> phonemes -> readout by program, and encoded a couple short songs for the pitches chosen by the vocaloid. Jun was a Sona for a long while, but when Diamond signed the contract, they saw the chance to become, and promised Jun that they would be made into their own, independent android once Diamond had perfected their own body.
Diamond is closest to their mom, a Cassandra Jasper, who lives in the city with their younger brother, Onyx Jasper, and calls Diamond about once a week to ensure they haven't died. While Diamond does feel bad that their mother is aging and going to die eventually, Cassandra won't hear a word of Diamond's plan to become Steel, and refuses Diamond's repeated offers to assist her. She does, however, indulge Diamond's curious mind, having Diamond do repair work when she needs it, and giving Diamond old appliances instead of throwing them away. She also keeps Diamond up to date with her favorite soap operas - whether Diamond wants it or not, though Diamond typically does enjoy hearing about it while they work.
Diamond's second-closest person is their boss, an Ahmed Jenkins, a second-generation immigrant who owns the plant Diamond works in, and makes a point to have lunch with Diamond once a month to discuss the factory and Diamond's assessment of efficiency and improvements. While not particularly outspoken, Diamond and Mr. Jenkins do enjoy wordless smoke breaks on long nights, which Mr. Jenkins always punctuates with a jocund, "Good to hear you speak your mind" before they go inside. Diamond gets the joke, but doesn't quite have the heart to actually speak their mind about their Goal to Mr. Jenkins, and is running out of time to explain as they near closer and closer to their goal (which they will inevitably need time off work to recover from).
And the last closest-person to Diamond is Onyx, their brother, and a somewhat-estranged one at that. They came into conflict when Diamond moved out, Onyx asserting that Diamond abandoned the family, something Diamond vehemently denies to this day. While Onyx has sort of come around on this particular topic, he's still resentful that he is "stuck with all the work" of taking care of Cassandra, despite being happy to do the work himself - he just believes that Diamond has an obligation to at least contribute. Having witnessed Diamond's willingness to drop even work to come assist Cassandra, though, this wound has been somewhat mended, though not completely, as Diamond still doesn't show their face for holidays like Christmas and Thanksgiving unless it's an emergency, leading both Onyx and Cassandra to recruit neighbors with broken appliances to lure Diamond home for the holidays. It's worked, and Diamond is onto them, but they can't deny the holidays are nicer with friends.
Diamond's father is best not spoken about. For what words will suffice, his first name was not King, even if it was the only name he would answer to.
Cassandra did her best to nurture her strange child, her kid who didn't speak until age 3, and did so in full sentences. She became a single mother when Diamond was 5, and her husband's grave was shallow and unmarked. Onyx was only months old, so Diamond appointed themselves a helper of the household, beginning their fascination with machinery as they sought to blindly imitate their father's most admirable traits, disassembling the fan for their room and almost putting it back together correctly. As their technical skill improved, they became more withdrawn, comfortable in the miasma of machinery moreso than with people, seeing social skills as a means to the end of returning to their chosen kin of machine-kind. They were 15 in 2004, and upon learning of the supernatural through the internet, they set about trying to confirm the existence of the soul through research alone, and by 18, they began working on the first drafts of who they would become. Jun, their vocaloid, was created from their voice in 2009 at age 20, recorded on the best speaker-made-microphone they had available to them.
They graduated from a technical college for engineering, and immediately sought a job in steel manufacturing so they could see firsthand what they would need to achieve their goals. Mr. Jenkins took them in, and after noting the way that Diamond worked slavishly on their own designs on the work floor, repurposed that drive for the factory, allowing Diamond to build whatever the hell they wanted, provided it didn't leave the factory floor, and it improved factory function. Diamond, unable to leave well enough alone, happily accepted.
Diamond has not had time for love of people. Not others, and not fully themselves, either. But Diamond has loved machines, and the shape of the Complete, a shape they view themselves as trying to achieve. A microwave can be finished, it can have everything it needs, it has a final form and a defined form factor. Not so with humans.
Diamond did have an admirer, back in high school. A quiet boy, Johann, with a fascination with firearms, who took Diamond to the shooting range on weekends where they customized and test-fired various firearms on-site in Diamond's mobile-workshop of a van. On the last day of summer, just before college would start and separate them from their high school relationship, Johann took Diamond to the range, and gave them a handgun - unremarkable in appearance, but expertly balanced for Diamond's hands specifically, sporting a custom reload mechanism that they designed together for hands-free reloading. Once Diamond accepted, Johann shooed them off the range, and then disappeared from Diamond's life, moving away to college in another state.
Diamond is most afraid of being unable to finish their self-project. In that way, they are afraid of mind-corrupting and -altering effects, as they are not too fussed with the body being harmed, since it's just an opportunity to replace things without Diamond having to fight their own self-preservation instincts. They are instead afraid of mind alteration because of what it means for the shape of the "truth" of themselves - they know damn well that how they feel could be rewritten by the occult, and they are horrified of the idea that it could change things. Now that they can see what they "really" look like, they're starting to become afraid of looking at themselves in the mirror, afraid that what they see will change, afraid that what they've been working towards for years will end up a dysphoric disappointment.
Having said this, I think them watching someone else change in this way would not be nearly as threatening to them psychologically, that they could watch with a kind of pride as someone closes the gap between themselves and... well, themselves. I think this will be challenged completely if they ever see Jun change, and Diamond won't understand why it's so petrifying to watch happen.