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💛- Pollux is the only character where this answer is interesting, ignoring Cy who is a self insert and speaks/is learning everything that I speak or am learning.

Pollux's family from his father's side are immigrants from another region of Flatland, hence the odd last name "Schwarz" that sticks out from between "Campbell", "Carver", "Smith" etc. Schwarz just sounded cool, there is no actual reference nor reason there. I do not think that they speak German or anything - German and English don't precisely exist in Flatland.

But different languages do exist, though mostly have been assimilated or only differentiated by their sound, and Pollux knows some of his family's mother tongue, though Hunter is much more fluent and Madelyn learned a bit of it to impress him. Pollux possibly quite regrets not learning more of it. Might know some bits of old dead tongues and ways of writing because he loves history! On that topic, he probably talks very awkwardly, more so than originally depicted, because he really loves those ancient styles and they mess with how he talks.

He also learns [post-TAoB] English and many Planiturth languages, also learns or helps create Exwhylian/Ypwherenian/Zetwhatian and other dimension's languages, and Thought Beyond Words for communication with those languages he doesn't know. He especially likes to learn old languages even now, even if that means older than time - would be funny to see the lil guy suddenly speak in the eldritch whispers of Voidspeak.

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🍰- Pollux loooves anything he can bake. Well, except most normal types of cake. He prefers stuff like tres leches, cheesecake or flan over regular cake. He loves those! He also loves making them. While he'll gladly eat a slice of birthday cake or one or two cookies, I think he really prefers things with some moisture to them. Crunchy or dry things sadden him. He's a chocolate chip cookie enjoyer too. But I don't think he's fond of adding fruit to desserts unless its blueberries.

Bill isn't my oc but I gotta note that this guy considers pieces of drywall, entire handfuls of specifically powdered sugar, and gasoline as "desserts".

Madelyn loves anything with strawberries [sadly Pollux can't stand them] and possibly the same cake preferences as her son. Hunter thinks sugar will rot your beak.

Lastly, Marianne actually really enjoys just fruit of any sort with unhealthy amounts of chamoy. Yeah she's the kind of girl that eats flaming hot takis like her life depends on it.

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🤔- Haven't thought of this much, considering they're. Literally just. Shapes. I figure that Pollux isn't great at talking- he tends to try to talk too fast or say many things at the same time or accidentally misuse words because he's attached to old language styles. So that's one thing. Also, when he's alone or nervous he likes to repeat words that sound nice to him, or the letter x (remember he loves how it sounds?) or just clicking. Even now that he can speak with words, sometimes he mumbles things to himself in Flatland's language, that nobody knows anymore except for him, so chirping and stuff.

I picture Youko would figure out what Hiei is from his powers (fire demon) + the wrappings he was in (Imiko) + where they found him (somewhere fairly close to the Yuki-onna’s rumored location), but initially he would have absolutely no interest in returning Hiei there, nor even letting Hiei know what he knows.

Youko always struck me as very self-serving and the sort who would enjoy having information on others, and not caring enough about other people’s traumas to go out of his way to address them (it’s just not a very legendary demon bandit thing to do, imo). I also always got the impression Hiei didn’t tell his gang of bandits about wanting to take revenge on the Ice Maidens, which is a HC I usually carry into this AU.

Basically, Hiei doesn’t share his ultimate goal, and while Youko knows what Hiei is and can correctly guess what he wants to do, he offers no help because it doesn’t particularly benefit him.

That’s not to say he doesn’t grow to be invested in Hiei’s well being, in his own detached demonic way, but Hiei’s proclivity to recklessness and getting in fights just for the fun of it is more pressing than Hiei’s potential identity crisis. Youko focuses on curving Hiei’s thirst for fighting and teaching him relevant survival skills (largely in “deliberately put you in measured danger so you can deal with it on your own” ways, with Youko stepping in to help when necessary but making it look like that wasn’t the main goal).

I think the first time they’d actually talk about Ice Maiden Island would be the moment Hiei loses his tear gem by indulging a risky fight with a demon, and Youko finally calls him out on his recklessness, throwing the fact he’s lost his sole prized possesion in his face (by this point Youko would expect Hiei to have more sense). Gives them a chance to fight it out, and Hiei’s goals can be addressed in a way that can be dressed up as beneficial to both parties: Youko just can’t risk having another right hand man who’s going to risk the bandits’ goals for personal gain, so he’ll help Hiei find Ice Maiden Island.

Before that, though, Youko tries to pull of a heist on spirit world and kicks the bucket, and Hiei shows us that no, he didn’t learn a single thing about recklessness, because he goes right on to Shigure when Youko’s gang disbands to get himself the Jagan. He’s going to find his tear gem, the ice maiden village, and Youko, while he’s at it (because he refuses to believe Youko would really let himself be killed).

So the story flows more or less like canon, largely because I like Fox Dad as a passive AU rather than a strong divergence (though I’m certain heavy divergence would be fun).

ACTUAL RELEVANT BIT TO THE QUESTION:

When Hiei and Kurama meet up again, Hiei has already found and visited the Ice Maidens and learned about Yukina (Hiei learns about Yukina from Rui, and was unaware he had a sister before it was so revealed to him; Youko wouldn’t have had a way to guess Hiei had a twin sister, or at least I see no mention of Imikos necessarily having to be half of a twin pair).

Last time Hiei admitted his goal of looking for something importnat to Youko the bastard went and died, so he doesn’t tell Kurama about having a twin sister he’s trying to find when they meet again in the human realm. Hiei merely lets him believe he’s still only hunting for his tear gem, to which Kurama offers his help (Hiei helpfully offers, “try not to die this time”).

The ruse is up when they have to rescue her from Tarukane, but since Hiei doesn’t want/can’t tell Yukina he’s her brother, there’s not much Kurama can do anyway, except looking after her and teasing Hiei about their relationship.

….Which Kurama already canonically does! ◉‿◉

(Go to 0:50; if you haven’t seen the english dub for YYH I hope this’ll sway you into it.)

Or that time they were all playing cards in the Dark Tournament hotel room and Kurama teased Hiei about why’s he not joining them?

Kurama as he interacts with everyone on Team Urameshi is very doting and thoughtful, and of course for Fox Dad I’d like to see him be extra thoughtful of Yukina, but I think what he canonically acts like towards her would already be pretty accurate. Kind and thoughtful, but not overly cloying or too obvious about considering himself a guardian figure to her just because there’s teasing and then there’s “give away Hiei’s secret without his consent.”

(Have I mentioned how easy it is to re-interpret canon Kurama/Hiei interactions into Fox Dad and how much I love that? Because I do. Kurama’s such a Mom Friend.)

Sorry again for getting so long-winded! And I’ll throw in a reminder of “this is just a silly AU and if you want to grab it and do your own thing with it, feel free”!

Want to picture Youko is more involved in Hiei’s plight, so he takes him to the Ice Maiden Island and adopts Yukina before she has a chance to wander off on her own in search for Hiei? Let that foxy grampa raise himself twins!

Want Kurama to appoint himself Yukina’s guardian to make up for his abandonment of Hiei and be attentive to her (in obviously parental ways which Kuwabara nonetheless perceives as potential rivalry, despite Yukina treating Kurama like a respected elder)? Go for it!