Rukia came from the humblest of beginnings (as much as can be expected for a dead person), but quickly rose to the top of Soul Society hierarchy upon her acceptance into House Kuchiki. She has never forgotten the amount of effort and sacrifice it required for her to reach that level, though, which is a rather essential moral component for anyone in a position of guardianship.
Her zanpakuto, Sode no Shirayuki, is the first one to appear in Bleach, although it isn't released so much as used to transfer Rukia's reiryoku into Ichigo Kurosaki. It's ice-based shikai is overpowering, but in a completely different way from most other zanpakuto in Soul Society — the elegant snowy frills, the flowing ribbons, the sparkling white. Are they ideal for each other, though?
10 Perfect: Rukia Is Emotionally Unresponsive
Given her orphan childhood and street-living, Rukia finds it hard to share her emotions with other people, no matter how close they are to her. This trait is first observed by her original captain, Juushirou Ukitake, as he notices her inexplicable remoteness during her time in his squad.
Sode no Shirayuki, being the embodiment of coldness (and, therefore, aloof) matches this tone of Rukia's persona. In fact, this is probably why her zanpakuto spirit evolves into its final form.
9 Not Perfect: She Isn't That Cold
At the same time, Rukia doesn't actively prevent friendships from happening, for instance, her rapport with Ichigo, Orihime and Renji is practically unbreakable (not to mention her past relationship with Shiba Kaien.)
Zanpakutos are necessarily mirrors of the Shinigami soul, so this really doesn't explain why Rukia, after her power-scaling stint in the Soul King's realm, comes back with powers that help her freeze things faster. Has she let go of all hope for warmth in her future?
8 Perfect: Her Training Under Shiba Kaien
Rukia's early years within Seireitei were marked by confusion and feelings of unbearable anxiety (what with her adoption by the scary Kuchikis.) It is Shiba Kaien who creates a safe space for her, in a way, by becoming her informal sensei: he teaches Rukia the art of the sword and how to commune with her zanpakuto, offering tons of little wisdom-nuggets in between.
Sode no Shirayuki's natural shyness would have been a lot more problematic for its wielder if Kaien hadn't been around to help with the basics of shikai manifestation.
7 Not Perfect: Sode No Shirayuki Is Far Too Beautiful
Although Rukia is a very sweet person for the most part, she is not aesthetically comparable to her weapon. This isn't to malign her appearance, but the fact that Sode no Shirayuki has been called "one of the most beautiful zanpakuto spirits" in existence doesn't really make sense.
Well, Rukia does undergo a drastic costume, hair and makeup upgrade when her bankai is activated, but it's not the same thing. Sure, it's not like zanpakuto are always similar to their Shinigami in this context, but Sode no Shirayuki has received such an epithet for a reason.
6 Perfect: They Are Equally Respectful
Rukia is well known for her pathetic deference towards her elder brother, Byakuya, but she is polite to everyone else as well (as a rule.) She cannot tolerate Ichigo's general lack of decorum, often telling him to behave more properly, and according to the situation.
In the Zanpakuto Unknown Tales arc, Sode no Shirayuki displays an identical level of courtesy, referring to both Senbonzakura, Byakuya's Zanpakuto spirit, and Rukia with reverential honorifics.
5 Not Perfect: What's With All The Dances?
Being one of the most awkward/graceless characters in the story, Rukia's pairing with the exquisite Sode no Shirayuki — whose every move bustles with an impossible elegance — is quite ridiculous.
She has no talent for the arts, her crayon scrawlings being the best evidence, but her zanpakuto requires all kinds of gorgeous twists and whirls in order to trigger certain shikai moves. Some no mai, San no mai, and Tsugi no mai are quite mesmerizing, but what kind of villain would fall for that, really?
4 Perfect: At Least They're Friends
Rukia's relationship with Sode no Shirayuki may not be ideal, but it's certainly superior to many other shinigami-zanpakuto pairings, such as Ichigo/Zangetsu or Kira/Wabisuke.
They have not experienced any kind of rift in the canonical storyline, unlike Renji's many issues with Zabimaru and Kenpachi's blatant refusal to even consider that his zanpakuto might have a living spirit trapped inside the blade. Sode no Shirayuki has never lied to her, given her incorrect information, or tricked her into believing it was someone else.
3 Not Perfect: It "Kills" Her To Use Sode No Shirayuki
Rukia's fully transformed shikai is currently capable of reducing the temperature of her surroundings drastically, although this is only possible by reducing her body to well under freezing point first. According to her logic, this form of "cryostatic" thermal control supposedly "kills her," (which is why she claims she cannot feel Äs Nödt's The Fear.)
However, this technique is extremely morbid, not to mention that a zanpakuto that causes "death", however short term, is antithetical to Rukia's role as a Shinigami.
2 Perfect: Works Like A Charm On Her Final Boss
The creepiest of all the Quincy, Äs Nödt, fails to affect Rukia's shikai, as stated above, getting himself (temporarily) frozen solid when she dips to Absolute Zero. Nevertheless, his Vollständig is overwhelming even for her, which is why Byakuya's timely arrival and sage advice help her gain the confidence required to release her bankai.
Together, Rukia and Hakka no Togame, which is in fact the actual name of her zanpakuto, utterly decimate Äs Nödt, converting the demonic Tatarforas into an impotent ice sculpture.
1 Not Perfect: Extremely Dangerous Bankai
Although Rukia's bankai reveal was one of the most exciting moments of the last arc, Hakka no Togame is far more harmful to the wielder. Rukia does not have the space to make a single error in controlling it, because the "cold" emanates from herself as much as from her zanpakuto.
She cannot keep it activated for extended durations, as her lack of experience could result in her own destruction. For instance, she must compulsorily defrost at a glacial pace to avoid shattering into a million flesh crystals.