The child is born in July, under the rising sun that never touches the newborn's skin, hidden in a hospital room and welcomed by the yellowish artificial light that lights the place. The mother sighs as the father coos, and the child is handed to them in a white towel, the clean and pale skin almost bright in comparison to the light, the small tuft of stark red hair like a bloody birthmark over the baby's head. They coddle it, as the mother names it after the father, and the father smiles brightly and tears up a little when the peaceful child seeks the mother's breast, warmth and food. Even so young and still blind, the kid finds what it seeks easily, latching to the nipple at the first try.

That day, Jack Fenton Junior is born.

When the baby is eight months old, Maddie accidentally leaves the TV on while the child is playing with the ghost-shaped plushies Jack personally made. She doesn't even realize it, absorbed in her work, trying to think of cleaner ways to power the house (and there's a vague thought about mixing that idea with their ghost research, but she can't quite figure out how), until her mind is rudely brought back to the present by a high-pitched sound.
Her son is laughing.
Then she notices the music, and the TV that's been left on, and she smiles. On the floor, Junior claps happily and shrieks in joy as the smooth sound of the piano is accompanied by the trumpet's rough tones.
That's how the nickname 'Jazz' is born, and afterward it just sticks.

When Jazz is two, a new baby is born.

The first time Jazz sees the little bundle of rosy flesh and white blankets the three years old frowns, and pinches a round cheek, hard.
The newborn cries in protest, and the parents swarm around it, the beginning of a scolding already coming out of their lips. To Jazz's frown is added two little arms crossing in a sulk, and the only words that leave the small mouth in answer is a sullen "I wanted a baby sister".
Both parents look taken aback, but then little Danny needs the soft voice of his mother and the calming and repetitive movements of arms and legs, and Jazz misbehavior is thrown to the sidelines.
Jazz doesn't like boys, and hates the baby boy who's stolen mom and dad.

During the next nine months, Jazz ignores the child in the crib and concentrates in grown up books, learning the A B Cs alone and then passing from picture books to books with only some pictures.
But dad and mom look only at Danny, and Danny's annoying gurgling, and Danny's poop problems, because Danny is dumb and doesn't tell them when he needs to go like Jazz does.
Jazz doesn't like Danny-boy at all.
So one night Jazz sneaks into Danny's room, and stares at the sleeping child through the bars of the crib. Ugly, is the best way to define the small thing inside the crib, so Jazz tells him just that, and then how dumb it is too, for not telling dad when he needs to go.
The child keeps sleeping peacefully, and Jazz finally huffs and leaves just as quietly.
Jazz keeps visiting Danny at night from then on.

Jazz knows mom and dad are worried about Danny when he turns one year and still hasn't talked ever once. But Jazz doesn't knows why dad and mom are worried, so the natural step is going to the books.
When they prove much too difficult, Jazz goes to the librarian, a nice old woman who's always there for help, and asks her to teach Jazz how to understand what the books say, and almost throws a fit when the librarian tries to explain a simplified version and leave it at that.
Jazz wants to know.
The librarian finally accepts and explains concepts and terms and spells out for Jazz new words as best as she can. Jazz understands a little better, after that. And a little bit of worry worms its way inside.
The nightly visits keep on, but now Jazz brings books and reads them out loud, softly to not wake their parents. And so Jazz tells Danny about princes and princesses, dragons, ghosts and children lost in dark woods.
One of those nights, Jazz announces that "she's the princess, so Danny shall be the prince in distress", and when her eyes go back to where Danny lies, he's looking at her, gaze intense and enraptured. Jazz blinks, and keeps reading the story, spinning in her lips the tales of Princess Jazz's adventures and her brave fight against the Dragon Ghost.
In a fit of inspiration, the story ends with Prince Danny being rescued and the curse upon his lips being removed, giving him back his speech ability. Danny's asleep by then and Jazz feels pretty proud, even as sleep makes everything blurry out once back in bed.
It's the day Danny makes one year and two weeks (Jazz has already learned the numbers, of course. And the calendar in the Fenton kitchen has had the individual days been crossed out for a while now, not that the adults have realized it yet), that Jazz thinks that, maybe, having a baby brother isn't so bad.
Because the feeling that hits deep like soda bubbles inside the stomach when Danny looks up, smiles wide and happily cries "Jazz!" is almost too much, even as Jazz starts to jump and scream in glee, quickly followed by Danny's own screams.
When Maddie and Jack run inside the living room, where their kids were supposed to stay calm as they played and read, respectively, it's to find Jazz hugging Danny, who keeps laughing and repeating the same word.
It could be said that's the day Jazz finally falls in love with him.

Jazz and Danny become like nail and flesh. Jazz is overprotective, always there to take care of the "baby brother", and always carries a book under the arm for when Danny wants a story. Jazz helps Danny learn the A B Cs, though it takes him much longer than it took Jazz, but his pronunciation is a little better, and Jazz feels as proud as their parents are.
The smile goes out of Jazz's face the first time someone says "how much of a good brother he is".
When Danny is learning to say dad and mom, brother and sister, Jazz almost hits him for pointing and saying "brother!", voice full of bouncy merry. Jazz does pick up the book and throw it away in anger though, and Danny is left looking confused, upset and a little scared.
That night, Jazz visits him and whispers to not call her "brother". Danny nods, face serious and meaningful, and then asks with a little voice what would Jazz like to be called then.
"Sister" is whispered in the night, as two children seal their secret with clapped hands, secured by the dark and protected by the moonlight entering through the nursery's window.
Jazz feels a touch of magic vibrating under their skins, the moon brightening a little as energy flows between them, and knows their promise is forever.
Unbreakable.

Jazz is four when she tells her mother she's a girl. Her mom looks surprised, which is silly, because Danny never had a problem and has taken to correct anyone who ever dares to call her a "brother" in his presence, broadening his list to "boy" when he realized the word upset Jazz just the same than the other one. So she doesn't understand why it seems to be so difficult to understand for grown ups.
Finally though, mom seems to understand, and she takes her in her arms and hugs her, in a way she hasn't ever done before, and Jazz wants to cry and laugh a little at the same time. Mom then proceeds to make them lunch while she talks about buying Jazz dresses and shoes and headbands for her hair. Jazz likes the idea, and plays with strands of it as she hums at her good sense of methodically slipping away from the hairdresser's clutches for the last months.
When dad comes back home, the notice is met by less shock and more joy, and dad spins her around until her laughter is so loud the neighbors next door complain, and hugs her and starts babbling about all the father-daughter stuff they can do now while proudly calling her his 'Jazzy-princess'.
For the first time in very long, she feels completely loved and safe, and she's the first to grab Danny's little hand and proudly announce that he already knew, much to their parents' surprise and amusement.
(Apparently, babysitters didn't take it well when one of their little charges started correcting the gender of their sibling.)

Epilogue.

For as long as Danny remembers, Jazz has been Jazz, his overprotective older sister who got annoying when she became a teenager. Danny remembers her all-knowing mind being the most obvious of the reasons he adored her during all his childhood, only close second to how she always was there, even when mom and dad weren't. Jazz always was the one who protected him from reanimated food, who was there to go back home with him when their parents were busy, who helped him when he struggled in a class, who asked for the identities of the bullies who would paint his skin with blue and purple until everything hurt.
(The only time he told Jazz who they were, she sent them to the hospital and almost got expelled herself. That had been in sixth grade, and the kids that had followed them from elementary to high school still left a path for him or his sister, afraid even though it had only happened once.
Danny hadn't told her again, realizing it had been his fault his sister had been almost expelled for beating his classmates to an inch of their lives.
Jazz, on her part, had sworn to never let herself lose control like that, and had stuck to the reputation of Perfect Student from then on.)
Danny knows Jazz hadn't always been Jazz or, more accurately, that Jasmine hadn't always been the name in her birth certificate. His parents had seen to fix that early on, thankfully, and Danny hadn't minded moving to Amity at the age of three. Best of all, he'd met Tucker in pre-k, so it all had advantages for everyone: mom and dad had a big enough basement for their lab, Danny had made friends and no one had known Jazz from before.

When Jazz turns eighteen she finally gets the bottom surgery that'd been postponed for years as a result of both the lack of savings and Jazz's own decision of leaving it until she was a legal adult. The days after the operation are spent with her in bed, laughing and enjoying the company of family and the close circle of friends who know about it, and occasionally chiding Danny for the new set of bruises caused by ghost fighting.
On a memorable (if better forgotten, due to the highly embarrassing contents) occasion, Danny phases through the walls in an impromptu visit to find Jazz checking out her new assets. He learns to use the door and knock before coming in after that.
After she can stand and walk, Jazz insists in buying herself a new pair of swimsuits, and spends the time it takes to the stitches to dissolve sunbathing on the roof with a book on her hand, when she isn't studying or helping with ghost hunting. Danny stays the crap away from the roof during the day, not eager at the prospect of seeing her sister naked, thank you.
Jazz gets out of it a nice tan, and the rosy scars seem less visible like that. Danny jokes that it's not like anyone would pay enough attention to see a couple of tiny scars in the middle of a good shag, and Jazz cuffs him in the head because apparently being a pervert with no filter for sex commentaries comes with being a teenager.
Jazz starts college feeling, as she fondly says, 'complete'.