The minute Kushina is born, everyone knows she is going to be a troublemaker.
She is loud and she seems to have a permanent mischievous smile. It is only expected that she go on to be an absolute force of nature when she grows up. Uzumakis are whirlwinds. But for all that predictability, there is one thing that stands out.
No one seems to expect her absolute adoration of her cousin Mizuki. Mizuki is not quite forceful in the same way - more timid than brash. She feels so much instead of defaulting to logic. It is not necessarily a disadvantage in this world where you have to trust your gut to survive.
At first, it seems like the two children don't have much in common.
But if you see them together, how vibrant they look, how alive, you'd understand.
Consequently, it is of no surprise at all to anyone when the two chose each other for partners when asked. Uzumaki shinobi come in pairs and really was there any doubt as to who they'd choose?
True, they seem like an unlikely pair but they seem to understand each other to a level no one else does. People get close but the trust and respect that ties them together is too deep for anyone else to grasp. Still, they have other friends, they don't keep to themselves all the time. Mizuki is never skittish with only one or two or even three people (and Kushina doesn't count, they're practically attached at the hip).
The only thing Kushina doesn't like is that her partner never joins her pranking. She'd be an absolute genius at it! Kushina can't count the amount of times Mizuki has suggested improvements to her ideas. The first time Kushina properly managed to pull one off, without getting caught, Mizuki had smiled at her in private and suggested improvements.
She got caught on the next assisted one she tried but it was worth it because all she could see was orange. Besides, punishment ranges from extra stealth training to better seals - depending on how you get caught. Of course, Mizuki always gets dragged along because "partners support each other 'ttebane!". If Mizuki is particularly grumpy, Kushina promises to take her for food afterwards. Both Kushina and Mizuki adore Ramen although Mizuki always chooses the blander ones. Ah, well, something needs to compensate for Mizuki's whiplash personality. Kushina steadfastly ignores the fact that she gets riled up even faster. Kushina's already energetic, it's only a little away.
Mizuki on the other hand goes from tired to snarky to kind to outgoing and insane and then it starts all over again - and not even always in the same order. Honestly, sometimes she's too much for even Kushina but that's okay because she knows Mizuki needs her alone hour once a day to not get too extreme and she doesn't usually mind when so Kushina can go spend some time (alone) with other people.
(Sometimes it feels like there's a missing Mizuki-hole near her but Kushina can deal with it. Mizuki goes, shuts herself off but she always comes back. Besides, it's not like she does anything interesting. Kushina used a spyglass on her once and all she was doing was splashing with the waves, climbing the trees or simply lying flat out somewhere. Kushina is so much more interesting, so she has no doubt her partner will come back. She will not be beaten by a beach. It's not even alive!)
Kushina learns to deal with it and gradually begins to enjoy some alone time too. Mizuki and her are both intense and sometimes it is nice to take a little break. As Mizuki likes to say "too much of a good thing is not good". Kushina knows Mizuki is good and she wants to spend time with her but she understands what she means - she shouldn't limit her allies.
Her other relatives (the rest of the island) aren't Mizuki but they are interesting in their own right so she doesn't feel as much when Mizuki leaves - she will come back, even if Kushina has to drag her so she might as well make use of the time.
She learns so much in that time.
Mizuki and Kushina are only just starting to mature so their chakra is extremely flexible right now.
Kushina's leaning towards her chakra chains so she spends a lot of those times with the boats. The people of Uzushio are not stupid - chakra and jutsus are used for destruction and healing - why can't they be used for creation? The normal issue is that they don't last long enough but they are known for their sealing and they can do some very interesting things.
On their very first mission, Mizuki took down three bandits by activating one seal. One! If Mizuki didn't become a seal master, Kushina was going to eat her hair. And that would be sad because it was such a useful weapon! No enemy ever expected hair to be used like rope. A bit of chakra had helped but she wasn't going to tell anyone that. Especially not Mizuki. She wants some secrets for herself.
Honestly, if Mizuki actually tries to remember what people tell her or she notices, she'd have a lot of blackmail material. Unfortunately, the girl simply doesn't care enough. There are very few things the girl actually does care about and Kushina is glad that she is one. Otherwise it would be like unrequited friendship.
They would never have this relationship.
Kushina is still glad that Mizuki approached her. She'll deny it out loud but she hasn't forgotten that it was that way around. But she was the one who sat with her the next day. Mizuki had looked so surprised that Kushina couldn't help but laugh. Halfway through a sarcastic comment, Mizuki's mood completely flipped when she saw Kushina's face and it had turned into a "Do you want to be my friend?".
Kushina still doesn't know why her family are just realising that her friend isn't as shy or "good" as they had thought.