Little White Mouse

Biodata

Power Progression

Background / History

Laboratory Origins

Xiao You originated among the white mice Yui specially bred for biomedical experiments. These mice normally lived for less than a year and had genes described as ninety percent similar to those of humans. Yui transfused her own blood into this particular mouse through the Blood Exchange Technique, fooling the Caged Bird Seal’s bloodline detection and allowing it to be applied. The transfusion was expected to leave the mouse alive for only two days 18.

Yui subsequently induced a deathlike state with a drug, causing the Caged Bird Seal to register brain death and disappear. An electric shock reactivated the mouse’s brain, and she awakened successfully. Yui kept her for observation because she had survived numerous experiments and appeared unusually intelligent 19.

Three days later, Yui’s laboratory Shadow Clone discovered that the mouse could speak. The mouse attributed her new intelligence to memories that appeared after she was knocked unconscious, but could not verbally disclose their contents. Grateful to Yui for what she considered a second life, she accepted a bloodline contract and accompanied the clone to investigate the original Yui’s absence 22.

Attempts to Free Yui

At Yui’s sealed room, Xiao You identified the confinement barrier and temporarily opened a vortex through which the original and clone could communicate. The effort exhausted her, leaving her severely emaciated. Yui supplied chakra, restoring some of her weight, before she fell asleep for several days 23 24 26 28.

After awakening, she learned the Summoning Technique to help retrieve Yui’s imprisoned body. When Ikuko intercepted the clone, Yui sent Xiao You away to obtain blood and find Yamada Hideko. A locked laboratory door prevented her from collecting the blood, and her attempts to ask passersby for Hideko’s whereabouts caused a public disturbance 28 29 31.

The Military Police detained her, and Captain Chengdou suspected she might be a foreign spy’s summon. Ikuko identified her as Yui’s summoning beast, then captured her and applied a Contract Seal. Xiao You mentally called for Yui, broke the seal with golden chakra, and escaped through Yui’s summons. Once reunited, Yui named her Xiao You and declared that they were family rather than master and servant 32.

Separation and Reunion

Yui’s experimental escape through the Summoning Technique left Xiao You behind in the sealed room. Spatial turbulence displaced Yui by roughly a year, during which Xiao You remained confined, spending her days eating and sleeping 33 35 37.

While exploring the poisonous blue forest, Yui considered summoning Xiao You but decided against exposing her to its dangers 34.

Yui eventually summoned her to Shikkotsu Forest and explained the unintended separation. Xiao You’s distress subsided, and she immediately asked whether Yui had been hurt. She then met Katsuyu, who welcomed her as a companion 37.

Travels and Poison Sealing

Xiao You accompanied Yui through the cave passage leading out of Shikkotsu Forest. Although her voice trembled in the darkness, she denied being afraid of the dark when Yui teased her 75.

When poisonous purple mist entered their cave, Yui told Xiao You to hold her breath and left her behind while investigating its source. Xiao You worried about Yui’s safety but accepted her reassurance. The mist came from the wounded Hanzō, whose summon Ibuse was absorbing the leaking toxin 76.

During the encounter, Xiao You befriended Baby Ibuse. When Yui retrieved her and left, the young salamander tried to follow, but Ibuse pulled him back. Xiao You promised that they would meet again and insisted that even a friendship formed within a second was still a friendship 78.

After Yui escaped her fight with Hanzō, Xiao You warned her not to remove the poison seal on her injured back until the wound healed, lest the poison enter her bloodstream. Rather than discard the poison, she proposed shrinking the seal and relocating it to Yui’s palm. She also transferred additional poison from her own seal, producing a painful procedure and leaving a mouse-shaped mark on Yui’s left palm 81.

On entering Iron City, Yui registered under Xiao You’s name rather than her own, privately reasoning that the mouse was entering the city too 87.

Shared Name with Yui’s Split Body

Yui later gave the name Xiao You to her Evil Ghost Mask–wearing Split Body so that Nai could distinguish the two versions of Yui. Nai explicitly recognized that this was already the little white mouse’s name. The masked swordswoman subsequently called Xiao You is therefore distinct from the mouse 147.

The Byakugan use, swordsmanship, elemental sword energy, and battle against Taiichirō attributed to that masked Xiao You belong to Yui’s Split Body, not to Little White Mouse 147 150 152.

Land of Whirlpools

After Nai awakened, Yui introduced her to Xiao You and Shobue in Shikkotsu Forest. Xiao You subsequently traveled with Yui and Nai; on their journey toward the Land of Whirlpools, she rode on Nai’s shoulder and joined Nai in commenting on Yui’s unusual teasing of Uesugi Yuri 131 171.

While accompanying Nai’s covert activities in the Land of Whirlpools, Xiao You used a newly learned sealing jutsu to conceal their presence. The effort exhausted her, and she angrily questioned what Nai was trying to accomplish 194.

Nai later kept Xiao You confined in a cage and took her along while preparing to disclose the Nine-Tails jinchūriki’s location to the hostile ninja outside the country. Xiao You resented this treatment and wanted to reach Yui but was forced to accompany Nai 200.

When the dispute reached Yui, Xiao You accused Nai of lying, while Nai claimed that the mouse was acting. Unable to determine who was telling the truth, Yui reassured Xiao You that she would discipline Nai later. Nai then admitted her arrangement with Mito to obtain a tailed beast, and Yui chose to respect her decision 204.

Nai subsequently returned Xiao You to Yui during the aftermath of the Nine-Tails incident 227.

Black Mist Space and the Mirror Passage

During the later exploration, Xiao You mistook a floating, transparent soul body for a ghost. Terrified, she hid in Yui’s sleeve but continued warning Yui to be careful. Yui reassured her and ultimately restrained the figure using the Spirit Transformation Technique 300.

When a passage between the Black Mist Space and the Red Ant Desert was discovered, Nai considered sending Xiao You through it as a test. She initially postponed the idea because she did not know where the mouse was 394.

Yui then located Xiao You sleeping deep inside her sleeve. Her steady breathing and twitching eyelids led Yui to wonder whether the purple mist had affected the mouse and returned her consciousness to her original body; this remained a hypothesis rather than an established explanation 395.

A subsequent summoning experiment brought Xiao You to the Red Ant Desert. At first she lay limp and appeared asleep, prompting Nai to question why she had not awakened as the scroll suggested. Shortly afterward, she struggled violently, jolted awake, and was confused by the sudden change from darkness to the desert’s bright sunlight 396 397.

Xiao You later tried to enter the Mirror’s black veil. Nai caught her by the scruff and questioned her, but Xiao You insisted that she had decided to go in. She reasoned that the scroll’s safe passage suggested that she could also cross and regarded herself as a consciousness that ought to return to its original body. Nai released her abruptly, causing her to fall through the veil before she was ready; the Mirror briefly vibrated as she disappeared 399.

In the Black Mist Space, Yui found Xiao You curled up with her eyes closed after emerging from the passage. Concerned about her condition, Yui watched her and compared Nai’s written account with what Xiao You had told her. Several minutes later, she observed the mouse’s body twitching through her Byakugan 400 401.

By the later encounter with an unfamiliar humanoid figure, Xiao You was awake and able to question Yui about her grave expression. Yui gave a vague answer rather than explain her concern for Nai’s and Orochimaru’s original bodies. Xiao You chose not to press her and returned to checking the body 403.

Appearance

• White fur and jet-black eyes: Her ordinary appearance contrasts with the golden light produced by her sealing power 23.
• Fluffy tail: Yui finds this especially appealing; Ikuko later recognizes it as a clue that she is not an ordinary mouse 22 30 31.
• Tiny body and short legs: Small enough to ride on Yui’s shoulder or sleep in her pocket 24 33.
• Golden markings: When using her barrier technique, luminous patterns spread across her fur and her eyes shine gold 23.
• Former forehead seal: Initially bears a blue-green Caged Bird Seal, which disappears after Yui’s successful experiment 18 19.
• Sleeve-sized companion: Frequently shelters or sleeps inside Yui’s sleeve, including during travel and the exploration of the Black Mist Space 97 300 395.

Personality

Xiao You is grateful and deeply attached to Yui. She accepts their initial master–servant bond because she believes Yui gave her a second life, volunteers to learn summoning without hesitation, and blames herself when the rescue plan fails. Yui’s decision to treat her as family gives her a more personal sense of belonging 22 28 32.

She is proud of her inherited sealing knowledge but easily discouraged by her limited strength. Affection embarrasses her, while perceived abandonment causes considerable distress. She also dislikes being handled roughly and becomes distraught when a police officer steps on and dirties her fluffy tail 22 23 28 31 37.

She forms friendships readily and defends their sincerity regardless of how briefly she has known someone, as demonstrated by her immediate attachment to Baby Ibuse 78.

Her concern for Yui coexists with a mischievous streak. While modifying Yui’s poison seal, she teases her and delays the most painful part of the transfer, provoking playful threats of retaliation 81.

Xiao You can be frightened by unfamiliar surroundings and supernatural-looking creatures. She denies fearing the dark, but later openly panics at the apparent ghost and hides in Yui’s sleeve while still warning her of danger 75 300.

She becomes openly resentful toward Nai when confined, treated harshly, or drawn into secretive schemes. Nevertheless, she is willing to make her own decisions about danger, insisting on entering the Mirror passage after reasoning that she should return to her original body 194 200 204 399.

Abilities & Skills

Awakened Memories and Speech

Her unexplained memories provide both human speech and specialized knowledge 22 23.

• Can converse with humans and other summoning creatures 22 37.
• Cannot directly recount the memories’ contents: attempts produce squeaks instead of intelligible speech 22.
• Yui suspects inherited memories and an ancient bloodline, but Xiao You’s exact nature remains unidentified 22 24 37.

Sealing Jutsu

Xiao You understands sealing mechanisms beyond her current ability to dismantle them 23.

• Identifies Yui’s barrier as an intermediate chakra-absorbing seal with primary and secondary points 23.
• Explains that attacks strengthen the barrier and excess absorbed energy is redirected to its secondary point 23.
• Opens a temporary communication vortex for at most fifteen minutes, rather than fully removing the barrier 23.
• Suffers severe exhaustion and weight loss after sustaining the vortex; requires chakra and prolonged sleep to recover 24 26 28.
• Can break Ikuko’s Contract Seal with golden chakra, despite being unable to undo the confinement barrier 23 32.
• Learns a presence-concealing sealing jutsu that hides herself and Nai from detection. Its use produces golden light and leaves her panting with exhaustion 194.

Poison Sealing and Transfer

Xiao You can modify seals containing poison and transfer stored poison between them 81.

• Warns that removing Yui’s poison seal before her back wound heals could release poison into her bloodstream 81.
• Shrinks the conspicuous seal and relocates it to Yui’s left palm, preserving its usefulness as a weapon 81.
• Transfers additional poison from her own seal into Yui’s seal through a painful procedure 81.
• Leaves a mouse-shaped mark on Yui’s palm and cautions that careless use could poison Yui herself 81.
• The stored Ibuse’s Poison Mist later helps Yui disrupt Black Bear’s sense of smell and poison it; Yui remembers the ability after noticing the mouse mark 313 314.

Bloodline Contract and Mental Contact

Her bond with Yui is a bloodline contract rather than an ordinary summoning contract 28.

• Formed by consuming one drop of the original Yui’s blood 22.
• Supports Yui’s use of the Summoning Technique to retrieve her 28 32 37.
• Allows her mental cry for help to reach Yui 32.
• Enables her to sense Yui’s presence; she notices that connection disappear during Yui’s induced deathlike state 40.
• Remains usable in the later summoning experiment involving the Black Mist Space and Red Ant Desert, although Xiao You’s initial sleep and subsequent awakening leave the relationship between her body and consciousness uncertain 396 397.

Summoning Technique

Yui teaches Xiao You the hand seals boar, dog, bird, monkey, ram to prepare for rescuing her original body 28.

• Learns to use the technique within three days 29.
• Lacks sufficient chakra for the intended rescue without Yui’s assistance 29.
• Requires Yui’s blood to summon her because Yui is the master in their bloodline contract 29.

Spatial Passage Experience

Xiao You participates in tests of the Mirror passage connecting the Red Ant Desert and Black Mist Space, but is not established as possessing an independent ability to create or control such passages 394 399 400.

• Chooses to enter the black veil after observing that a scroll could pass through safely 399.
• Describes herself as a consciousness that should return to its original body; the underlying mechanism is still being investigated 395 399.
• Appears asleep after emerging into the Black Mist Space and later resumes conversation with Yui 400 401 403.

Relationships

• Yui Hyuga — Breeder, former experimenter, contracted partner, and chosen family. Yui names her Xiao You because the name sounds like the first syllable of her own, and asks her to stop calling her “Master” 18 22 32. Their partnership later includes poison-seal modification and dangerous spatial investigations. Yui repeatedly protects her from environmental hazards and worries about her safety after she crosses the Mirror passage 34 76 81 400 401.
• Yui’s Shadow Clone — Discovers her speech, forms the initial contract using the original’s stored blood, and teaches her summoning 22 28.
• Yui’s Split Body, also called Xiao You — A separate version of Yui adopts the mouse’s name to distinguish herself from the bronze-masked Yui. The two are not the same character, despite sharing a name 147.
• Ikuko Hyuga — Pursuer and captor. Tracks her footprints with the Byakugan, but refrains from killing her because Yui would blame her 31 32.
• Yamada Hideko — Intended source of rescue assistance; Xiao You searches for her on Yui’s instructions but does not reach her 29 31.
• Katsuyu — Fellow companion of Yui. Welcomes Xiao You warmly, then startles her by revealing a slug-like form beneath her human transformation 37.
• Nai Hyuga — Traveling companion with whom Xiao You develops a strained relationship. She rides on Nai’s shoulder and helps conceal their presence, but objects to Nai’s secrecy, confinement, and rough treatment 131 171 194 200 204. Nai later handles her during the Mirror experiments and drops her into the passage after she insists on entering. Although Nai worries about Yui’s reaction if Xiao You is harmed, no separate summoning contract between Nai and the mouse is confirmed 397 399 401.
• Baby Ibuse — Quickly becomes Xiao You’s friend during the cave encounter. Xiao You promises to meet the young salamander again when Ibuse prevents him from following her and Yui 78.
• Hanzō and Ibuse — Their poisonous mist threatens Xiao You during the cave encounter. The poison later stored in Yui’s mouse-shaped seal becomes a useful weapon 76 81 313.

Notable Quotes

“You awakened my memories. That's as good as giving me a second life.” 22

“Waaah, Yui, I thought you'd abandoned me!” 37

“Hmph, so what? Even if we've only known each other for a second, we're still friends.” 78

“Nai Hyuga, put me down. I'm mentally prepared. I'm going in!” 399

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