A/N
I've made some changes and added new traits to vampire-human hybrids in this story.
What I changed: When hybrids reach full maturity, they gain strength equivalent to that of full vampires. (In the Nahuel section of the Official Guide, hybrids are described as weaker, even when they've reached full maturity.)
What I imagined: Adult hybrids appear to be around 25 years old. Since they are born supernatural and never have a chance to live a normal life, I felt it would be unfair for them to be frozen as eternal teenagers if they reached full maturity before age 20.
Billy Black Jr. was born when Nessie was five (physically 17), and Elizabeth Black was born when Nessie was six (physically early-20s). Apparently, their names came from Billy Black Sr., Jacob's father, and Elizabeth Masen, Edward's mother. The Cullens were uncertain at first if Nessie and Jacob could have babies (they finally accepted the fact that Jake and Nessie are meant for each other, romantically) or not. They decided to seek out the infamous Joham guy before the Volturi eliminated him. Huilen and Nahuel kindly showed them the directions.
Fortunately, Joham was still alive and well with his three daughters. He was wary at first, but he fell for Carlisle's passionate facade—he acted like a mad scientist, which surprised everyone ("Well, he somehow possesses a mad scientist in him," Edward said later, "If not for his conscience, he would have already been the greatest vampire-mad-scientist, dissecting every 'criminal' executed by the Volturi.")—and told him everything he and Serena (who was now studying with her father) knew about hybrids. Surprisingly, the hybrids could give birth with humans.
"I, along with my sisters, started having periods when we were five years old," Serena explained matter-of-factly. "My father wondered if it meant I had reached my childbearing age. Unfortunately, I was the only hybrid we knew, so we tried with humans. That was actually fun for me, having sex with my meal." The Cullens tried not to wince at this statement. "I miscarried a lot. Three times, to be exact. I usually drink blood, so I didn't know the baby needed human food. Technically, the baby is one-quarter vampire, so the human side is larger. I succeeded on the fourth try. This time I only ate human food. It was a boy, but he didn't have venom, unlike Nahuel. And he was... mortal."
"Mortal?"
"Yes. The one-quarter vampires are mortal, growing and aging like humans. Their growth is rather faster than humans—they reach maturity in 15 years—and aging is slower, but eventually, they die of old age. My son... he died when he was 115. One-quarter vampires are a bit stronger and faster than humans, but not much. They don't have bloodlust, although they crave raw meat a lot. They heal fast, both physically and pathologically. Their offspring are just... humans. I once met my grandchildren, but they were nothing more than humans. So I think it's safe to say that one-eighth vampires are humans." Serena paused for a moment. "Oh, I forgot to mention. If the little one—sorry, her name is hard to pronounce—is planning to give birth, she'd better hurry. When we reach full maturity at seven, the period stops. That means we become sterile. Forever."
"Firstly, I'm sorry for your loss."
"That's fine. It's long ago now."
"And secondly, do you mean the hybrids' fertile period is only two years? From age 5 to age 7?"
"Exactly."
"How do you know when you've reached full maturity? Is it determined by the cessation of the cycle? But that doesn't apply to boys. Perhaps it's the seventh birthday?"
"Ah, no. You know, hybrids may appear weaker and slower than vampires, but not anymore after reaching full maturity. I call it Hardening. I experienced a mysterious fever somewhere after my seventh birthday. It lasted about three days. When I recovered, I found myself as strong, fast, and durable as my father. Of course, needles can still pierce through me if forced powerfully enough, and my heart still beats, and so on, but from then on, hybrids can match vampires evenly."
"I call it Eclosion rather than Hardening, though," Joham interrupted his daughter. "Because it's like hatching from a cocoon and reaching the final stage."
Serena snorted. "I'm not a butterfly, father. I prefer Hardening. Besides, insects become fertile after eclosion, instead of sterile. It's completely opposite."
"Well, it doesn't matter," Joham shook his head. "It's not like we can publish research papers in a journal like Vampirism or something."
So everyone was worried when Billy and Liz were born, knowing from Joham and Serena that they would be mortals.
"But there was something we all overlooked," Jacob said, during the Cullens' 'history' lesson.
"What was it?" Chris asked.
"That I'm a shapeshifter," Jacob laughed, recalling the memory.
They thought it was Hardening at first when Billy's fever never ended for days. But then they remembered that Hardening only happens to half-vampires, not the mortal one-quarter ones.
"Wait," Carlisle suddenly said one day, when Billy was unconscious from the fever again. "I think... I know what's going on with him."
"What's that?" Jacob asked frantically. He and Nessie were barely holding onto their sanity.
"His scent is changing. Don't you feel it? Oh, right. The smell is only unpleasant to full-vampires. What I mean to say is that Billy is turning into a shapeshifter. No, that's inaccurate. He was a shifter to begin with. We didn't have to worry about his mortality!"
To everyone's relief, Liz was also a shifter, although being a girl, like Leah.
"So, if someone mentions the Black pack, it means the three of us. I gave up the Alpha of the larger pack, passing on the position to my Beta," Jacob continued. "So, now there are three packs. Uley pack, in La Push, Clearwater pack, somewhere near there, and Black pack, mine, forever wandering with the Cullens!"
"So, what's the Channeling, then?" Chris asked again.
"Do you remember everyone else's gift?"
"Of course," Chris rolled his eyes. "because I have a perfect memory! Edward is a Mind-Reader, Alice is a Future-Seer, Jasper is an Empath, Bella is a Mental-Shield, Nessie is a Projector, and Billy is a Physical-Shield."
"I'll explain the Channeling myself," Liz intercepted her father and the young hybrid's conversation. "At first, well... it was when I was a year old, according to mom."
At first, everyone thought Liz's gift was similar, or the same as Nessie's gift. That was because Liz projected her thoughts directly at her mother, by touching her. But she was different. Once she touched someone, the 'connection', or 'the Channel', which they now all call it, remained, forever, regardless of physical distance. Furthermore, the projection of thoughts was mutual, like having a phone call. They later discovered that only sounds could be delivered through the Channel, so that was another difference with Nessie or Edward's gift, as theirs can convey or receive images as well.
"It's most similar to Alpha connecting, my dad thinks. Like Alphas 'thinking' to themselves, the people currently Channeling can only deliver thoughts they want to deliver, unlike the invasive 'collective monologue' among the one pack. In addition, if one is unwilling to chat anymore, the person can shut down the Channel." Liz frowned. She preferred her gift more, over the collective monologue when she is in the wolf form. "Growing up, I gained more and more control of my gift. Now I can create something like a group chatting room using the gift! I call it Group Channeling. Would you like to try?"
Chris nodded, dumbfounded. There was so much information to absorb.
-Liz: Group Channel set. Everyone hearing?
-Chris: I was shocked at first, but this is so fascinating!
-Liz: Thanks!
-Jasper: This gift really improved our safety, tactically. Until Channeling, Edward informed us what he'd heard via voice. That gave away his gift to our enemies, if not careful. But with this? We're almost invincible.
-Carlisle: Actually, we all know sign language, initially for communicating underwater, but sometimes used for conveying silently. But the problem was, many vampires thought the same thing! The sign language was easy to read.
-Edward: Now I can freely spy on any enemy I meet and tell everyone in the Channel.
-Chris: Do you have many enemies?
-Carlisle: Well, not really. The purpose of 'spying' is to see if there's danger or not, not necessarily for fighting. I try to form an alliance with everyone, or every coven.
-Emmett: But if they're hostile... We strike 'em down!
-Esme: Not so fast, Emmett! Be civil.
From that fateful meeting with the big family, Chris promised to keep in touch with them even if they move out. "I'm not going to join you, no offense, but because I want to see the world with my own eyes so far. Besides, I'm already connected to your Channel, am I not?"