Rukia is not only one of the main characters in Bleach, but eventually one of its strongest as well. While her powers as a Soul Reaper were hidden for a while thanks to her transferring her powers to Ichigo, she eventually gains them back and proves herself to be an extremely proficient fighter. Eventually, she becomes the Lieutenant of Squad 13, and at the end of the story, its Captain.
While Rukia is an expert user of Kido, her main strength lies in her Zanpakuto, Sode no Shirayuki. Sode no Shirayuki takes the form of a long, snow-white blade, and is often called the most beautiful sword in the Soul Society. As might be surmised based on its color, Sode no Shirayuki is one of a few Zanpakuto whose abilities revolve around ice.
Sode no Shirayuki's Powers
While it initially appears similar (though less powerful) to the other major Ice-Type Zanpakuto in Bleach, Toshiro Hitsugaya's Hyorinmaru, the mechanism for how Sode no Shirayuki freezes opponents is quite different. While Hyorinmaru freezes water, including the moisture in the air, and then controls the resulting ice, Sode no Shirayuki drastically lowers Rukia's body temperature, allowing her to freeze anything she or her blade comes in contact with.
Rukia only figures out the true nature of this power after extensive training. Initially, she doesn't seem to have any control over the extent to which her body temperature drops, or even the ability to notice it. Instead, she erroneously believed it was the blade itself that froze opponents. The attacks she uses for most of the series reflect this misconception. Some no Mai, Tsukishiro involves Rukia drawing a circle on the ground with Sode no Shirayuki. Any target unlucky to find themselves in that circle or above it are instantly frozen in a pillar of ice, which then shatters along with the victim.
Although Rukia typically uses Kido in long range combat, she also occasionally employs Tsugi no Mai, Hakuren, which involves drawing a circle and then stabbing the ground four more times. The punctures start to freeze, and Rukia can direct that energy forward to create giant avalanche of cold air and ice, which instantly freezes anything caught in its path. In a pinch, Rukia can also use San no Mai, Shirafune, which freezes the area immediately around her blade to extend it. She was able to kill the Espada Arroniero by sticking her broken blade up to his face, which stabbed him as it repaired itself.
Rukia's New Powers and Bankai
After training during the Thousand Year Blood War arc, Rukia learns the true nature of her powers and gains precise control over her body temperature. She managed to dispel some of the Quincy As Nodt's fear-based attack due to the fact that her body is technically dead when she lowers its temperature, because her molecular functions temporarily cease. As Nodt quickly figured out, however that Rukia's nerves remain intact, and therefore attacks which target her eyes work on her.
Rukia is now able to form ice shields with Sode no Shirayuki, as well as freeze the ground under her, making it rupture. She can also lower her body temperature to absolute zero, allowing her to freeze anything she or Sode no Shirayuki touches instantly. That said, she can only do so for about four seconds before her own body starts to break down, making it a very dangerous ability. She must also raise her body temperature back gradually, or risk permanent damage.
Rukia's Bankai, Hakka no Togame, is still a bit mysterious because it has only been used once. When using it, Rukia's hair and body turn completely white, and she is dressed in a pure white kimono, looking much like the mythical Yuki-onna (snow woman) of Japanese folklore. Hakka no Togame drastically expands the range at which Rukia's absolute zero temperature freezes things, making it an all-encompassing, instantaneous power. Byakuya commented that Hakka no Togame's power comes at great risk, as a single misstep could end Rukia's life. This is displayed when her blade-holding hand forms cracks after using it.
With Tite Kubo slowly writing new chapters of Bleach and the series returning as an anime later this year, fans are sure to see even more of Rukia's power in the near future, and perhaps more elaboration on the abilities of her Bankai. She may also get better at controlling the power of absolute zero with less strain on her body. Even with that handicap however, no one can say Sode no Shirayuki's power isn't frightening.