Author's Note: It took me so long to come up with a workable idea for this one, which made me double-mad because I love writing zanpakutou nonsense. I eventually realized it's because I like writing longform zanpakutou nonsense, slowburn zanpakutou nonsense, if you will. Anyway, here's a vignette set somewhere amidst my Heart is a Muscle series, somewhere around or after Call Me Back When the War is Over, which features Akon discovering that Rukia and Renji's reiatsu is weirdly similar and subsequently trying to study them like a pair of bugs. You shouldn't need to read that to enjoy this, though, just a general appreciation of Rukia being a huge troll.
Edit: Forgot to say earlier, but this was somewhat inspired by an interview with Kubo, where he was asked if Zangetsu (OG big-knife Zangetsu) is heavy, and he replied that you swing a zanpakutou with your reiatsu, not your muscles, and that Renji and Ichigo would each find each other's swords to be heavy. So that part, at least, is semi-canon.
The second Rukia saw Renji whip his arm back, she hit the dirt in a forward somersault, and then twisted onto her back.
"Bakudou Number Four! Hainawa!" she barked, just as Zabimaru's lead segment hit the apogee of its arc. The crackling yellow rope of reishi lashed securely around their spiky bits and then, instead of pulling it towards her, Rukia threw.
All manner of colorful language poured out of Renji as he went hurtling through the air after his hapless sword. Rukia gave a quick, vindictive jerk on her spell. Zabimaru did a quick mid-air about-face, abruptly retracting when Renji lost his grip on the hilt. The weapon hit the ground, embedding themself halfway up their picks into the soft dirt of the training field. A few seconds later, Renji hit the ground fifty yards away with a loud grunt and a small cloud of dust.
Rukia stood up and brushed off her hakama feeling very proud of herself.
"That was so uncalled for," Renji groaned.
"I have observed," Rukia announced, striding across the field, "that happen to you in actual fights. I wondered how it was accomplished, since my experience is that Zabimaru is outrageously heavy and difficult to deflect."
Renji sat up and flopped his forearms on top of his knees. There was dirt smeared across his face. "Yes?" he frowned. "And?"
Rukia stopped next to Renji's blade, standing up in the ground like a flagpole.
"But that's the nature of a melee-type zanpakutou, isn't it? It's heavy, but not to you, because you wield it with your own reiatsu. So I simply had to wait until you threw them in the direction I wanted, and I gave 'em a little extra push. Just like a hakuda throw!"
Renji attempted to rub the dirt off of his face and only managed to smear it cutely across his nose. "I can ground myself with my reiatsu before I extend Zabimaru, but I lose a lot of mobility that way." He sighed. "Chalk up one more thing I have to watch out for when I'm fighting you."
"What kind of fighting partner would I be, if I didn't spot your weaknesses and help you overcome them?" Rukia teased.
"You could have just mentioned it. You didn't have to throw me headfirst into the ground to prove your point."
"I didn't do it to prove a point. I did it because it was fun." Rukia glanced over at Zabimaru. "I bet I could pick them up."
Renji cocked one eyebrow skeptically. "Yeah, I'm sure you could. They're not too heavy to lift, they're just too heavy to use. Hisagi and I used to try to fight with each other's zanpaktou for a lark sometimes. And you know. Resistance training. The zanpakutou hated it and we always ended up injuring ourselves."
"You kept trying, though?"
"You know how we are."
"I do." Rukia made an expression of mock-hurt. "You let Hisagi hold Zabimaru?"
Renji shrugged. "Look. Go ahead and pick 'em up. I can tell you want to."
In fact, she did. Rukia stood up a little straighter, and squared her shoulders. She shifted Sode no Shirayuki to her left hand, and took a good grip on Zabimaru's hilt. It would have been easier to do with two hands, but that wasn't the point. She gave a strong jerk. The dirt around the base buckled a little. Rukia wrinkled her nose and tried again. This time, the sword shifted significantly, but still stuck in the ground.
Renji watched all this with calmly lidded eyes, not saying a word.
Rukia gave one more strong wrench, forcing a little extra reiatsu down her arm, and Zabimaru finally broke free.
"Nice job!" Renji announced, making no motion to get up.
Now that she had Zabimaru in hand, Rukia realized that carrying them presented its own problem. Rukia was too short to hold them vertically, but if she held it horizontally, the moment-arm of the heavy end segments was agonizing on her forearm muscles.
Renji continued to watch her in silent amusement. Bastard.
Rukia took a deep breath, and swung, heaving Zabimaru up onto her shoulder, the way she'd seen Renji carry them many times. It wasn't comfortable, but it was at least bearable. "There we go!" she declared cheerfully.
Renji let out a loud laugh. "You've proved your point, you menace, now give me back my sword before they bite you."
"You're such a worrywart!" Rukia chided, walking over to him anyway. "Zabimaru loves me!"
"The only thing Zabimaru loves is causing me grief," Renji replied, holding out his hand.
"Here, give her some attention before she starts getting jealous," Rukia said, pressing Sode no Shirayuki into his hand instead.
"Rukia!" Renji yelped. "Eep, she's cold!" He quickly rearranged his legs so he could set Shirayuki across his knees. "There you go, beautiful," he said gently to the sword. "I'm so sorry about her, but I'm sure you already know." He glared up at Rukia. "You can't just go around handing your unsealed zanpakutou to people!"
Rukia snorted. "She only pretends not to like you to piss me off." She swung Zabimaru down into a two-handed grip she could just barely manage. "Now let's see how much you like me."
"Rukia, you can't," Renji warned, although the seriousness in his voice was belied by the fact that he was trying to arrange Sode no Shirayuki's ribbon artistically across his lap. "Extending the segments is 100% reiatsu-based. They won't do it for anyone but me."
"We have pretty similar reiatsu, though," Rukia reminded him. "And Akon said it would probably continue to get more similar, because of the power of our phenomenal friendship."
"Akon said it would take a decade at least," Renji reminded her.
"He said it usually does. He also said that we're freaks. I'm gonna try to synch our reiatsu."
"That only works when we're not actually trying to do it," Renji pointed out.
Rukia cleared her mind and tried to think Very Renji Thoughts. Tall, she thought. Immovable. A heart of iron, the depth of the sea.
To her side, she heard Renji sigh, and felt his reiatsu come in underneath hers, a perfect two-part harmony.
Lend me your strength, Zabimaru, Rukia thought to herself. This isn't actually important, but I know you love a bit.
It wasn't exactly a voice, but something hummed back, two more parts to the harmony, one even deeper than Renji's register, and one above him, but below Rukia.
Rukia drew her arms back and slung them forward. "Howl, Zabimaru!" she yelled, for good luck.
The huge sword swung down, nearly taking Rukia's rotator cuff with it. Time seemed to slow down to molasses. Ponderously, the sword lengthened, like a Kuchiki aunt trying to to touch her toes. From the tip, propagating back to the tsuba, each segment stretched, reached, managing to pull a full millimeter apart from its neighbor. It hung in the air for the briefest of moments, and then, with an absolutely pathetic CHNK-CHNK-CHNK-CHNK, snapped back into place.
Somehow, the sword seemed to have gotten a little lighter.
"I told you I could do it," Rukia proclaimed triumphantly. "You saw it. You witnessed it."
"I don't believe it," Renji gawped. "I cannot- Rukia, that was amazing!"
"Thank you," she preened. "To be fair, I had the advantage of having seen it done before." With great care (and great difficulty, as well), she managed to get Zabimaru turned around so she could present them back to their owner hilt-first. "Thank you, sir, for the loan."
"Yeah," he snorted," accepting his blade as easily as if they were made of paste and paperboard. "Any time. I guess." He passed Sode no Shirayuki back, before hefting himself to his feet. "I hope you don't expect to see me do a Hakuren soon. I'm still in charge of piggyback rides, but the miracles are your department." He shook his head as he sealed his sword and sheathed it. "Fucking unbelievable, as usual, Kuchiki. Please don't tell Akon you did that."
"Oh, your poor ego is in no danger, I shall keep it a secret for now," Rukia announced primly, putting her sword away as well. "How's your hand?"
Renji frowned and looked down, stretching his fingers open. "My hand? Nothing happened to my hand. I landed on my head when you threw me."
"No, when I handed Sode no Shirayuki to you. You said she was cold?"
"Huh? Oh, no, I was just surprised!" He flexed his hand at her. "See? No harm done."
"Mm," Rukia nodded thoughtfully. "Kaien tried to pick her up once, you know. I had to take him to the Coordinated Relief Station. Third degree ice burns all the way up his arm. Happened in less than a second." She shot him her sauciest wink. "I told you she liked you."
Renji's mouth fell open. Rukia loved it when he made that face. He looked so stupid but also so very, very cute.
"Come on," she said. "Let's go get lunch. After all that hard work, I'm starving."