He spends most of his young life on the periphery of Mal, Evie, and Carlos's lives. Jafar isn't very interested in spending time with Maleficent and Grimhilde, especially in the weeks after the fourth of April, when he is rather swamped with dealing with a newborn (after the initial weeks, it becomes a question of pride- of proving that he didn't need help to take care with his son) and planning said newborn's mother's funeral.
Jay is a thief, which is fairly common on the Isle of the Lost.
He is also a lone wolf, which is not[1].
In spite of Jay's lack of presence in Mal, Carlos, and Evie's youth, Jafar is rather more important to the story of Evie, Jay, Mal, and Carlos than you might expect. For one thing, he is the one to suggest the "town halls" to get rid of Auradonian spies (he had, of course, been part of the figurehead of an authoritarian government for a while, and so had some ideas about how to deal with this one). This is important because the town halls were, as we've established, where our story starts.
For another, he is also behind the rumour about cannibalism that follows Evie around (and, to a lesser extent, he's also behind that one time that she actually ate human flesh).
See, Jafar ran the pawnshop at the centre of the Isle of the Lost.
Well, to call it a pawn shop may be a bit kind. Really, it was more of a Stolen Goods And A Couple Of Pawned Items shop. For all that Jafar had endlessly persecuted the pickpockets of Agrabah, it turned out that he was quite good at it himself.
The thing about the pawnshop is that it is the best place to go if you need anything. It's also the place trash goes to die. Once a month, Auradon sends a barge over to the Isle, with all of the scraps that they can't sell anymore. Fast fashion items that were produced too many times, the bread that grew stale before someone bought it, the parts of the meat the butchers can't sell to princes and princesses (anyone who sets aside of a portion of their product to be sent to the Isle gets a pretty heavy tax rebate, so the haul generally isn't that bad).
The contents of the barge are then rationed out to all of the households of the Isle, without much care for usefulness.
Once the barge is beyond the magical barrier again, the trading begins. There's a pinboard up at Jafar's shop, and if someone needs something, in particular, they can leave a note asking to trade (there are problems with this system. Specifically, Jafar tears down a lot of the posters, so that the original writers eventually have to come to him).
But many people will go straight to Jafar to trade. He's something like the hub of the Isle black market, and he's more likely to get you what you want, faster.
Even though Jafar's shop is supposed to be a pawn shop, a lot of items that generally wouldn't be found in a pawn shop end up there anyway. One of these items is the meat that butchers in Auradon struggle to sell; specifically organs- brain, eyes, lungs, heart. Jafar will generally trade for them because he can charge starving people quite a bit for any kind of food (the store doesn't have a freezer, though, and so the residents of the Isle of the Lost get very good at pickling).
This is all a lot of backstory for a very normal interaction. The Evil Queen had eaten venison heart more than once, by the time she was locked up on the Isle (most famously, the time she'd thought she was eating Snow White's heart). So every month, she goes by Jafar's place, drops off the two loaves of bread that she's allocated (she and her daughter are on a strict no-carb diet) and in return, she walks out with a jar of pickled hearts.
One day, someone sees her and spreads the rumour that they're human hearts that she's walking out with and that she eats them in a ritual to keep her and her daughter young (they spread this rumour for a laugh, but eventually it gets spreads so far and wide and taken so seriously that even they begin to wonder if it's true).
The rumour is actually, all-in-all, good for the Evil Queen. People stay away from her, worried that if they anger her, she'll cut out their hearts and eat them.
This fear doesn't keep them away from Evie.
By the time Evie is fifteen, she's smart as a whip and probably the best-liked person on the Isle. She plays her role well- she's kind and friendly to other women, and flirtatious but not wanton around men, and pretty enough that she always gets her way.
She's also friends with Carlos and Mal (because her mother likes to hang around Maleficent as if her power will rub off on them), so no one on the Isle is likely to try and take her title.
Evie is endlessly fascinated with them. Not in the least because they were all born at the same time- she's sure that there's something more to it than circumstance, no matter how firmly Carlos tries to tell her that, statistically, it's not unlikely for four children to be born at the same time, not when there are roughly three hundred and eighty-five thousand people born each day.
But where Evie is svelte curves and soft lines, Mal and Carlos are all strength and sinew and muscle and hard planes. And sometimes, she'll stare at them when she catches them kissing, and wonders if they can really be enjoying it as much as they seem {Mal tries to kiss her once, and it leads to the supremely awkward moment where Evie shrinks back and giggles softly (the awkwardness doesn't last long, and eventually it probably ties them closer together, but it does permeate the air around them until Carlos tells them to knock it off)}. What fascinates Evie about the three of them is the feeling of absolution whenever she is near them. They will forgive her for any sin, just as she will forgive them.
{In our universe, Emily Brontë wrote: "Whatever our souls are made of, his and mine are the same"- which is not nearly as romantic as you might think, but that's a story for another time. And besides, later in the same book, Emily wrote another line- "I am Heathcliff! He's always, always in my mind: not as a pleasure, any more than I am always a pleasure to myself, but as my own being"- that captured Evie's feelings on the matter much more accurately. She was not made of the same stuff as Carlos and Mal, but rather, she and Carlos and Mal were all one form of existence, tied to each other as inextricably as the names of romantic novels. Just like you never think of the name Romeo without Juliet, or the name Bonnie without Clyde, it was impossible to think of Evie without also thinking of Mal and Carlos}.
Evie, Carlos and Mal meet Jay for the first time at a town hall. The hall is just Coward's Market, an open-air piazza in the middle of the Isle that's usually filled with temporary stalls selling whatever is available. Mal is perched on a wall, Evie in her lap, Carlos leaning against her legs, when Jay hops down from a fire escape to sit near to them (not next to them, he's not an idiot, but there's not much space left in the square and especially not space so far from the hastily-constructed stage the alliance representatives are sitting on (Lady Tremaine, Maleficent, Hades, Ursula, Claude Frollo, and Dr. Facilier). Mal side-eyes him for a second but seems to decide that he's no threat {Carlos keeps a careful watch on him, though. He doesn't let his guard down, not around his girls}.
From their vantage point, all four of them can see Marissa, who is leaning against the broken statue of Belle and the Beast in the middle of the square.
Of course, Claude Frollo fucks everything up.
It's the first time he's ever been a representative- he'd gotten too evangelical for the Tremaines, with his eternal preaching of sin and damnation, and they'd turned against him.
"Brothers and Sisters, are we not the children of God?" he asked, spreading his arms wide to gesture to the whole crowd gathered in the square.
He was met with silence {mostly- "Some of us more than others," Jay muttered from where he was perched on the wall, watching Mal and Carlos out of the corner of his eye. The former snorted when she heard it, and the latter giggled, but neither acknowledged him beyond that}.
Still, Claude Frollo was undeterred.
"He made each one of us in His image," Frollo continued, "and as such, He asks us to respect ourselves, and so respect Him."
"Get to it," Maleficent snapped, examining her fingernails {she, of course, had not been formed in the image of God, who was currently using they/them pronouns anyway}.
"Each of our bodies is His body. And so I can no longer tolerate the blasphemy you commit silently." He'd worked himself into a frenzy now, shouting, red in the face, spittle flying from his mouth.
"In this gathering sit men who lie with men, sisters to lie with their brothers-" his eyes flickered unconsciously to Maleficent, and Mal wanted to bang her head against a brick- "and women who reject their ordained roles as wives and mothers and instead turn to Satan to grant them pleasures of the flesh."
Marissa the Spy had gone very, very pale. She was watching the scene play out in the same way one might watch a very public breakup between people one is not actually friends with.
"Our God is gracious!" Claude Frollo shouted still, raising his voice louder to be heard over the jeers of the crowd.
"He forgives the sinners, the heretics and the apostates, but you must repent! He has sent me here to save his children from damnation!"
"What the fuck is this, then?" Someone- Uma, Ursula's daughter- yelled back from their position in the crowd.
"You're too fucking late!" Another person agreed, and then no one could hear anything for all the voices clamouring to be heard.
Jay sighed, pushing himself to balance on the wall. This was, to be fair to Claude Frollo, how most town halls ended. But it was also the first time anyone had raised the rumours- about Mal and Carlos, about the Isle witches who could practise without access to magic- in a formal setting, which meant that the Auradonian government might actually feel the need to step in {they would, of course they would, because this is where the story really starts}.
[1]: There is a Silmarillion-style amount of background required to understand this. So, for a moment, forget Jay specifically, and let's talk about the Isle culture that he grows up in.
When the Isle of the Lost Solution (detailed in the second amendment of the Constitution of the United States of Auradon) was first enacted, the state of the Isle could most succinctly be described as chaos. Many of the prisoners had been very much dead hours before, and Auradon had not thought to explain to them why they were suddenly not. This resulted in a killing spree unlike anything seen before- some horrendous mixtures of killing for self-defence and killing for gain.
About twenty-four hours after the Isle of the Lost Solution was implemented, the government of the United States of Auradon broadcasted their first Public Service Announcement to the Isle, explaining the situation. Those who were still alive formed tentative truces and worked to bury the bodies they'd left as soon as possible.
Forty-eight hours after the Isle of the Lost Solution was implemented, the recently-buried began to dig their way out of their shallow graves. This led to the first two alliances (houses, families, coalitions; they had a lot of names)- those who had lived through the first forty-eight hours, and those who had not. It was generally assumed that people would offer aid to those who belonged to the same group as them.
Of course, it couldn't remain that way forever, or even for very long. Many of those who had survived were extremely powerful, once upon a time, and so found it difficult to work together, and so their group fractured- Maleficent, the Evil Queen, and Jafar stood together, and so did the Tremaines, Gaston, and Governor Ratcliffe, and so on. Hades immediately situated himself in a cave system and literally didn't speak to anyone else until he fucked Maleficent.
Similarly, some of those who had died upon being sent to the Isle made the choice to seek protection from other, more powerful villains. Claude Frollo appealed to the Tremaines as a man of the cloth. Mother Gothel occasionally took care of Mal, Carlos and Evie.
These alliances often reformed, too. Some shattered entirely, some switched sides, and so on. It was a nightmare to learn, but most residents of the Isle managed fairly well, or, like Jay, kept out of it entirely.