Yuyuyu
She walks up to this apartment after depositing the mail of the previous one and knocks. She puts on her best smile
"Letter for a Senko, from....Tetsurugi Kaguya?"
When let in, Senko will see a girl about her age, black silky flowing hair tied in a pony tail, wearing a pink kimono and wearing an umbrella on her back for some reason, holding out the letter.
"Hi, I'm Yuuna Yuuki Yukimura but everyone just calls me Yuyuyu, I'm a new genin. I apologize if the normal mail man isn't here today, his horse got injured from one of the village mines bringing in mail from outside and now my team is working to deliver the mail in his stead, I hope your day is going well."
The notes whenever they are read will say this:
Spoiler: Yuki Bloodline Theories
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(Senko I have taken care to copy these notes as best I can for your reading, I apologize but I do not know enough for my own liking myself due to various factors expounded upon in the research notes proper- Tetsurugi)

The Yuki bloodline is a curious kekkei genkai, scientifically speaking. From a purely non-chakra perspective ice is just solid water. The chemical composition is unchanged, merely its energy state from a higher to lower one. Many Water affinity users have argued that it should be possible for normal water jutsu to achieve such an effect without a bloodlines use, like manipulating the energy within the water to be pulled out, yet numerous tests with water jutsu have proven this false. Indeed the claim is that the kekkei genkai somehow combines water and air chakra to form its ice, only possible through its bloodline. Thus ice in theory adheres to classical elemental theory rather than chemical theory in some manner but we have yet to figure out why, just like all other combined element jutsu. The Yuki have claimed this in the past, but given the nature of shinobi their word cannot be taken as true, as all clans of the ninja spread rumors to confuse, embellish and mystify their capabilities and its hard to tell what is folk tale or fact about them.

This kind of research would be hard and dangerous enough given that I'm trying to research a foreign shinobi's bloodline and therefore have numerous disadvantages to actually finding anything out about it. Even in normal conditions for someone in my position, the most that ethically can be done is to investigate recorded second-hand accounts of their sightings and battles from other shinobi that have bothered to say, but even then my sources are limited as I only have ninja from the Sound to call upon, which given our geographic position is not likely to encounter a Yuki even in normal circumstances.
Recent events with the Bloody Mist's purge of all bloodlines from its village during the Third Shinobi War and thus the Yuki's escape has driven the Yuki clan into hiding. As a Kaguya who fled from the very same purge I know that some still exist, but they did not come with us in our exodus from the Land of Water. The fugitive clans were divided over whether to escape or to fight back from the shadows and the Yuki clan were on Kenzo Hoshigaki's side of continuing to fight against Yagura's regime. I know that there was fierce debate between Ryota Ishikawa and Kenzo Hoshigaki at the time, but over what is unknown as their conversations were private, but it led to Ryota leaving. As far as I know the Yuki Clan is still fighting from the shadows. We don't receive many reports about what is happening in the land of water, as the nation is more isolated from other and Yagura likes to keep a tight control over information and his oppressive policies economically benefit the Lands of Fire and Lightning too much for them to intervene in Water's troubles, as few companies wants to deal with Yagura at all.
Most Yuki therefore are unknowns, but there is one Yuki we know of in the modern day Suzume Yuki, Kenzo Hoshigaki's right hand woman. Said to be a brilliant scientist and jounin level ninja interested in chakra theory herself and also known for being a jutsu thief. It is said in rumor that she can somehow make people show off their techniques in some way, that she has some power to mess with your mind and what few reports we have suggest that her ice mirrors sometimes show other people within them like some sort of film, but this could be exaggeration or lies. What is known for certain is that shipments of various supplies along the eastern coast sometimes sport residual ice from her leading the attack but we're still figuring out if the patterns of her attacks are different or similar to Kenzo's own.

What is generally recorded about their abilities is the famous ice mirror jutsu, known for how strange it is, but due to the fact that the user can move incredibly fast between mirrors, pretty much instantaneously, getting near an ice user in combat is pretty much suicide as there is more than instance of teams of otherwise competent shinobi suddenly dying from a slit throat when such mirrors appear. Most shinobi keep away rather than try to investigate due to this ability to create an environment of pretty much instant death. You get caught in that, your already dead. Recommended method of investigation is to send in clones and keep your real body away.
However this is just one technique of the ice user arsenal, one often used but it would not surprise if the Yuki clan has a small library of ice jutsu they keep to themselves and use the ice mirror jutsu as just its most famous one or public face to hide the full capabilities of the element. Indeed, there are legends of older era Yuki who have use their element differently, such as Komoyi of the Frozen Touch who rivaled the Hyuuga clan in taijutsu prowess during the First Shinobi War as he was able to freeze peoples blood or even their heart. Or Minuko the Razor Flake who was said she could kill with a mere snow flake honed to razor sharpness and came to be known as the Second White Death during the Second Shinobi War. Of course the First White Death, Turume Yuki during the Warring Clans Era was even more terrifying who is said to have wielded some kind of jutsu to induce hypothermia over entire battlefields and send blizzards at his foes.
The Yuki clan seems to seek out arctic or cold-adapted animals to make summoning pacts with, from arctic foxes to polar bears to penguins, seals, walruses, moose, birds, and the like to have synergy with their ice jutsu. This makes sense as few other ninja would seek out such summons and they probably have their own unique list of summons they keep in mind. Ancient Yuki during the warring clan era according to myth and legend even made pacts and summoned Yuki-Onna, the yokai from which they take their name. However no modern Yuki has ever demonstrated this ability and many question if Yuki-Onna even want to be summoned as the few attempts to reach out to that realm has ended with frozen shinobi, so we assume they want to remain isolated, so whether they can or ever could summon them remains ambiguous.

Now to get down to the actual theories of how the Ice Bloodline could work, is that while its not known for certain as all clans keep their bloodlines details secret to preserve its strength, there are a few theories of how it works:
-The Heat Manipulation Theory:
this theory tries to integrate the scientific and chakra sides of how ice works to form a cohesive mechanics of how it works. the theory goes that wind jutsu isn't actually "Wind" but rather the manipulation of energy with air as the medium they are limited to working within. combined with water and they are able to manipulate the energy state of water to somehow drain out the heat to make ice, but this theory has problems in that where does the heat go if the user has control over it? do they just let it dissipate into entropy? do they not know they could possibly suck up the heat to warm themselves? and it doesn't explain why or how the mirror jutsu works, unless they can somehow turn into energy, manipulate the energy of their reflection?

-The Chakra Elements Theory:
This theory focuses on the fact that this ice is made of or involved with chakra and that chakra has physics altering properties, reasoning that it instead functions as a material broadly similar to ice but not quite real ice. The theory goes is that all the ice is made of chakra and thus is Chakra-Ice, or C-Ice, which explains why water jutsu users can't make it, because its not actually ice, and chakra water isn't actually water, the problems with this is that water jutsu users can manipulate real water just fine and so can ice jutsu users, so its inconclusive.

-The Classical Elements Theory:
The next theory is that Chakra is actually a deeper more basic level of reality than atoms and thus that Chakra Ice is actually a more primal purer expression of ice than chemical ice thus explaining why ice can be manipulated yet why this ice has more properties than chemical ice.

-The Pocket Dimension Theory:
One of the crazier theories focuses on the ice mirror jutsu technique, claiming that their ice is actually crystallized pocket dimensions that they can go into, then transport to and from each other because its actually a space time jutsu that takes the appearance of ice. This doesn't really explain why its water and air element though, because space time jutsu doesn't really have elemental properties, and this theory is only here because no one knows if thats how the mirrors really work and its possible thats is a space time jutsu.

Of course there is a lot of debate over this topic as to how the Ice Bloodline works could provide numerous insights into how heat and the changing states of matter work due to the common nature of water and its vital functions to all life and its interaction with temperature, as well as how to approach a user of this bloodline as someone who could manipulate such things would be much prized for certain kinds of research in: chemistry, marine biology, hydrology, studying the weather and climate, human anatomy, potential medical applications, food preservation, heat regulation, and various other fields we do not have time to list here, not to mention the interesting contribution to the study of chakra one could make by finally having a willing combined elemental bloodline user show their abilities openly.

Further research will be needed, the logical course of action would be to find an actual Ice user of the Yuki Clan and figure out some way of negotiating them to give up their secrets for science, but given that would be interacting with a missing-nin and therefore a criminal at this current point in time especially one currently fighting an oppressive regime of a foreign country, it raises ethical questions that I'm not sure the shinobi world has answered yet. This issue should be treated with caution for the safety of everyone involved, including the researcher.

There is probably more technical stuff on other pages of the notes breaking it down even more but this is pretty much the gist of it. Yuyuyu seems completely comfortable, fascinated and interested by the person who is answering the door.
"You live by yourself? You must be good at taking care of yourself, real responsible."