[Warning: Spoilers ahead for chapters 569 & 570]
He found her, huddled in her room with at least three blankets wrapped around herself. Despite that, she still shivered. The sword in front of her caught his attention. It was Sode no Shirayuki, still in Shikai. It appeared to be covered in a thin layer of ice, which would match with the ice that covered the walls, ceiling, and floor of one of the training rooms some doors down; along with the chill that seemed to to flow throughout the manor.
"Rukia."
Her head popped out of the mess of blankets after an initial struggle. Patches of white remained in her hair, and some ice was stuck to her left cheek and underneath her chin.
"Yes, Brother?"
The room itself was not chilled, yet she spoke as though she were in a higher altitude, the cold air seeping the muster out of her voice.
"Why is the training room covered in ice?"
"I was practicing."
The simplicity of it caught his attention (although it was already more than caught by the ice). He stepped closer and looked down at her. The patches in her hair, he realized, were snow- and even as the patches fell off, they did not melt. At least, not yet.
With a steady hand, he reached for her face, bound for the ice on her cheek.
"Don't touch me!"
His hand froze, and his eyes suddenly jumped to meet hers. They were wide and slightly frightened. She diverted her eyes away and drew one of the covers closer around herself. "I haven't... returned to normal yet..."
He paused a moment; and instead of fraying her already-frazzled nerves by coming closer, he stepped back to retrieve another blanket from a closet in her room.
"What were you practicing?" He dropped to one knee to pick up her sword, using the blanket as protection between it and his skin. Even with it, the temperature was still cold to the touch.
"...Bankai."
He paused.
He took a few seconds to let the thought seep in, and then he released her sword and folded the small blanket into his arms. With soft steps, he brought himself in front of Rukia once more.
"Was it successful?"
Her eyes alone rose to his face. She hesitated, and her eyes wandered from his as she began speaking. "I couldn't... I had to stop early. The room was filling with ice and-and I didn't know how much farther it was going to reach. So I stopped before I could see all it could do."
"Did you sustain any injuries?" His voice carried no disappointment; instead it came across rather dull and stoic, just like his usual monotone voice.
"No." A bit of snow dropped from her hair, and she brushed it off her shoulder and away from herself. "Nothing aside from the cold."
"Are you saying you are not supposed to be covered like this?" he spoke, in reference to the snow and ice that was noticeable on her body, which he assumed had covered more before he arrived.
"No, I believe I am..." She trailed off a moment as a shiver went down her spine, and she pulled the blankets tighter around herself. "But I still need to get comfortable with the temperature."
"And what temperature does your Bankai reach?"
Her answer was hesitant, but she replied despite her desire not to. "Approximately -273 degrees Celsius."
There was no immediate reply from her brother. Not until she felt another blanket be placed around her, the warmth of it coming from the hold Byakuya previously had on it.
"Absolute zero. Molecules stop moving, and it is believed that there is no lower temperature." He stood directly in front of her and the mass of blankets. "I assume your body temperature is gradually increasing. What is it at now?"
"10 degrees."
Her body was above the freezing point already.
"For battle, I would recommend speeding up the warming process. However, I do not believe it would be safe to make any rash increases, as a sudden shift can be just as dangerous as keeping the temperature too low for too long."
She stumbled to speak; she did not expect him to be so nonchalant about her actions, let alone what the actions did to her own body. But she took his words to heart; being so cold that it impedes the ability to simply move would be an outrageous disadvantage in battle, especially at the rate she was taking to thaw.
"Thank you, Brother."
He crumbled, the ice having frozen As Nodt completely and utterly. The cold was slowly leaving her body, but she still felt some strain in her hand, some inconsistency as she worked through the thawing process: her hand cracked.
Her brother's hand reached for her own to cover it, giving heat to the area that seemed to be a different temperature than the rest of her warming body.
"Let it melt slowly, Rukia."
She was getting faster at the process; but just as he had explained before, speeding it up too much could be just as detrimental.
"Take your time."
She could feel her body finally becoming warm, just after she had truly begun to feel the coldness of her Bankai. She would be passed the immobile part of the attack in less than a minute. She let out a sigh and instantly saw her breath in the freezing temperature the area was now shrouded in.
Her brother spoke up again, in spite of the cold he, too, felt. And the swell that followed nearly popped her heart right out of her frozen chest.
"That is a breathtaking Bankai."
End
[Notice: The last quotes and battle scene come directly from the manga.]