Eirin

Eirin was found as an infant by monks of a secluded mountain order occupying the range between the Land of Frost and the Land of Lightning, who raised her for a few years before their destruction. There is no record of her origins. No family. No birth document. Nothing. She emerged from nowhere, which is, for Jashin’s children, entirely accurate. They had taught her their taijutsu style — the White Crane — and techniques predating the Sage of Six Paths. Eirin absorbed it all, how to be at peace with nature, how to remain kind in a cruel world.

The Senju led the strike that destroyed the order. The Uzumaki and the Sarutobi concurred. Three clans had reached the same conclusion: the monks preserved knowledge that predated the Sage of Six Paths, and that knowledge had no right to survive into the current age.

After the destruction of the monks, an Uchiha pursuit squad investigated the wreckage. A five-year old Eirin was found in the rubble by a twelve-year old Madara. She was still in shock of what happened, and Madara thought she was a ghost. Despite that, Madara handed her dried persimmons — hoshigaki — from his rations. This was the first and only kindness she ever received from shinobi for a long time. She looks up to Madara, without knowing his name, and he becomes the face of kindness she holds on to. Before he leaves to join the pursuit squad, Eirin thanks him and tells him his hair is "pretty and fluffy". Unbeknownst to her, this becomes Madara's primary reason why he never cuts his hair.

Rationing the dried persimmons Madara gave her, Eirin realizes nature chakra is keeping her alive as she wanders around the mountain range. Berry bushes would pop out beside her, animals would help her forage, and her small memories of her cooking for monks would form. She would try to experiment with cooking and her techniques. Sometimes, she would create autonomous snowmen to play with.

Matatabi finds her at some point, thinking a sage was in her territory, only to find a child causing a nature chakra embargo in a five-kilometer radius. Sensing that this child has a soul and chakra so strange — similar to the tales of the shamans that walked before the Ōtsutsuki even arrived on the planet — Matatabi takes her in. Eirin, thinking that she had finally found her mother, immediately bond with Matatabi. She has blue hair and Matatabi was blue, so why not? Later, she would be disappointed that she isn't going to grow up as a cat.

Because of this connection, Eirin is introduced to the other tailed beasts — particularly Shukaku, Son Gokū, and Gyūki — who become her mentor and foster family in various ways: Shukaku teaches her the concept of seals and tea ceremonies which she develops tremendously, while Son Gokū and Gyūki refine her taijutsu.

Following the path of kindness the monks taught her, Eirin develops the Golden Ratio Healing and Winter Lotus Gardens to heal both civilians and battlefields left in ruins by warring shinobi. She does not stay long, wanting to help as many people as she can, even if every single time she leaves, shinobi clans will fight and destroy the people and environment she has healed. She still remembers the boy who gave her dried persimmons, reminding herself that kindness exists within shinobi ranks.

As she travels along various nations and territories, her connection to the universe gets stronger. She senses bubbles of semi-sentient liminal spaces that came from a higher plane of existence. Realizing that when they brush against "base reality" they get destroyed, she makes it her goal to save as many as she could, developing the Hilbert Space Prism seal to contain each liminal space in their own pocket universe.

At some point, Eirin observed Madara Uchiha and Hashirama Senju fight once, from a distance. She found Madara’s Susanoo — blue — beautiful. She found Hashirama’s techniques an abomination. This encounter directly inspired one of her books: a ghost samurai protagonist and an evil forest antagonist. It became a cosmic horror classic decades later. She was published under a pseudonym, with the Fire Daimyo’s daughter as patron. She never received credit. She didn’t particularly want it. She just needed the money in a practical sense.

On another hand, Madara won that particular fight because Eirin's presence caused a nature chakra embargo and Hashirama couldn't use sage mode for a while.

She also glimpsed Madara’s hair and was jealous of its volume and presence.

On one occasion, Eirin saved an elderly noblewoman of the Fire capital from a threat the woman’s guards could not handle. The noblewoman survived. She has not forgotten. She treats Eirin, who neither requested nor expected this, as a daughter — commissioning her seasonal kimonos with the quiet authority of someone who has decided the matter is settled. Eirin accepts them. She does not know how to decline something given with that particular quality of sincerity.

When combat becomes unavoidable while wearing one of the noblewoman’s gifts, Eirin removes her kimono first. She folds it. She sets it somewhere safe. She then handles the situation in her undergarments with the same composure she brings to everything else. She has never explained this to anyone. No one has asked twice.

One evening, Eirin crashes into a cave after a gruesome fight with the Eye Hunger from the outer solar system. For several hours, she sleeps and regenerates. Eirin wakes up on a cot, with Madara Uchiha staring at her. She thanks him for not killing her while she is injured and asks if he needs healing. It is during this time she detects a parasite — Black Zetsu — deliberately stunting Madara's growth and suppressing his chakra at 40%. Asking if he wants it removed, Eirin's actions and Madara's agreement changes the course of history.