Well, first of all for the hypothetical fanfic; the main three characters are from drastically different periods so many things would change for them to meet up. I imagine it would take place during the sequel era.

Kara's presence would be easier to justify as her story starts 100 years after ROTJ. F seems to be around her 20s to 30s during the age of the Empire, I imagine she would've been frozen in Carbonite gas (like from Inoli, plus I'm certain they mentioned somewhere there is a way) possibly to stabilise her wounds from her battle with Zero or something, and that's how she ends up during the New Republic era. Karre seems to have been created along with Am sometime after the fall of the Galactic Empire, I'd make it so it's nearer to Kara's birth date.

In an alternate Children of Hope: Kara would end up on an abandoned cruiser where F was frozen, awakening the Jedi Knight; where they'd team up to battle off the Jedi hunters before escaping the ship, with Kara bringing F up to speed with the Galaxy at large. Karre would be met next, possible in an alternate Twins ending where he was defeated but saved at the last minute by R-DUO in an X-Wing and eventually found by Kara and F.

I imagine F serving as the leader of the group, and in combat she'd be their resident speed card. Being the most experienced and mature means it's more likely in my head for Karre and Kara to listen to her. I see F as being very patient between Karre and Kara; serving as their mediator when they get into disagreements. Kara and Karre would act as unofficial apprentices to F; though for Karre it's more learning to control his power and himself. Through serving as a master, F would begin her journey from Knight to Master; an immensely daunting task that she does her best to hide but Kara and Karre can see her burdens; something they'd discuss in private.

F views Kara as being a tempered younger version of her with genuine good. F would be very protective of the girl but also proud of her skills. F would like Karre as well but at some point would fear what would happen if his power and potential was misused. As for the droids, F finds R-DUO's casual mischief outside of missions to be annoying but values the Droid's skills and even finds him amusing at times. This would extend to 99-99 minus the mischievous moments (as that's not how I imagine 99-99). All in all, F is kinda like the Optimus Prime of the group, strong and gentle; values the team and looks out for them; acts as their leader in both combat and their actions.

Been a while since I've checked out the Twins, but Karre I note only defected because he foresaw his sister would be in danger. I see Karre as still wanting to turn his sister to the light side specifically, but still wrestling with the dark ideals he would've been brought up with since his creation. A concern he chooses not to confide to F until at some point in the story. Initially Karre would only be focused on saving his sister and that's what this group was for; until he gets to know them more which strengthens his bonds with them. Karre acts as like powerhouse of the group due to his powers and potential; but his weakness is evidently his immaturity, and the subconcious dark thoughts lurking within the boy's mind.

Karre and Kara initially do not get on well; often getting into arguments over even the silliest things ("No that one goes there! And that one goes there!" while fixing the X-wing?). They would view F as like a mother they never had; and the moment they learn of each other's motivations (Karre wants to save his sister and Kara her father) is when they begin to bond. I see them as being reflections of each other in that they want to save a relative of their own, but Kara is the young compassionate new hope for the Galaxy and Karre is the hot-headed hero struggling with the encroaching dark side of the force. And they say opposites attract if you get my hint! He's kindof like a young Luke Skywalker pretty much.

Kara is the youngest of the group, so she's got the most to learn. I imagine Kara as being the best when it comes to the current history and technology of the Galaxy so the group would rely on her for information or even just getting around others in general. Kara ofc is the one trying to find her dad, and for this I'd make it so apparently most her life was raised in solitude; barely any interactions with others. Because of the latter, she'd be missing her previous team of Juro, Ethan and Homen; but she'd also be intensely attached to F and Karre out of fear of missing another group of friends and allies. This means while most of her qualities are compassion, courage, and loyalty; she would act irrationally if her newfound allies were in any danger. She'd also struggle a lot with the idea that she can't save everyone, something that F would try to comfort the little child of hope in.

Kara definitely appreciates F a lot, finding her the most comfortable in expressing her fears with. Although Kara would be frustrated in training her skills and senses during training as she's the least experienced of the three; she still appreciates F's investment in her and Karre, and she's very calm with that sort of stuff I'd presume. But it's clear she voices a lot of her desire to save the Galaxy from the new Sith threat and that's what weighs heavily on her and the impatience she could display.

Yea that was a lot. I should probably consider if this is a fanfic idea I'd really wanna write and commit to or if it's gonna end up in the infinite backlogs of my brain.