"Oof!"
Startled, Sakura lurched forwards, tripped over her panties tangled at her ankles, and pitched face first into her futon. She scrambled to pull herself together and cover up, but by the time she dared peek into the hole, ready to fight, the White Zetsu had already vanished.
"Oi!" came an irate voice from down the hall. "Whichever of you bitches woke me up, you're done for when I get my hands on you!"
"You're in for it now, new girl!" the occupant of the cell across from Sakura's whispered loudly in her direction. "You've interrupted the boss lady's beauty sleep!"
But Sakura had bigger concerns than having angered the queen of the Blood Prison's so-called Garden of Lilies. If White Zetsu could infiltrate Hōzuki Castle using the Mayfly Jutsu, then not even this jail was safe from the Akatsuki, or more precisely, from the one White Zetsu truly served: Obito.
With over ninety percent of her chakra sealed, Sakura was dangerously exposed. She did not know why White Zetsu had appeared in her toilet hole, but she was counting her lucky stars that she had not voiced her plan to defeat Nagato and the Six Paths of Pain out loud…
"There's no time to waste," Sakura murmured to herself. "I need to be combat-ready by the time that thing returns… think, Sakura, think…"
Apart from Susano'o, Sakura's Mangekyō Sharingan abilities were either non-combative or supportive/defensive, so mastering Sage Mode was non-negotiable, but she was not willing to throw away her humanity to attain sagehood. Without the ability to summon Katsuyu, she at least needed someone to teach her… but where in Hōzuki Castle could she possibly find a master?
The answer, obviously, was from within.
Sakura sat in the lotus position, ignoring the irate voices of her fellow inmates. Deep within her breast, countless colourful lights seemed to shimmer in her mind's eye as she entered Breast Meditation and communed with the lingering consciousness imprints that she had acquired from her victims along with their breast chakra.
"So, at long last, you've come crawling back to me for help, Sakura-kun~…"
When Sakura opened her eyes, she found herself in a dark underground room. Fluorescent tanks filled with strange, misshapen creatures and body parts lined the space, the unmistakeable scent of formalin permeating the air. Before her stood a tall man whose face was caked in make-up…
The third member of the Legendary Sannin, the Giant White Snake of the three-way deadlock...
"…Orochimaru," Sakura said, holding her head high. "I've come to bargain."
Seeking help, the first person who'd come to her mind was Tsunade, but right now, the Slug Princess was the last woman Sakura wanted to see. By her own admission, Tsunade had never dared to train in the ways of the Sages of old… but Orochimaru had. He'd tried, according to Kabuto, but he'd failed. Still, failure was the mother of success, and right now, Sakura was prepared to take all the help she could get.
"What could you possibly have to offer me that I haven't already taken for myself?" Orochimaru said, grinning evilly. "Your bloodline has no more secrets before me."
"There is one thing you ought to know: there's a soft limit to how much foreign DNA my Kekkei Genkai can accommodate," Sakura said calmly. "Knowing you, your main body must be rushing madly across the continent to collect every Kekkei Genkai he can get his hands on, right? I'm already treading close enough to the limit as it is, so in my opinion, it won't be long before Orochimaru's cloned body experiences a total genetic sequence collapse…"
For the first time, Sakura saw a crack in Orochimaru's façade of invulnerability. His main body was rushing straight towards disaster, and he knew Sakura was not lying; chakra is the power to link all things, so in this meditative space, there could be no lies.
That being said, lying by omission wasn't technically lying. There was actually a way to increase the number of Kekkei Genkai Sakura could assimilate, but that involved raising her life level, her universal ranking as a lifeform, to the level of the Ōtsutsuki clan's… but Sakura wasn't about to tell Orochimaru that.
"What do you want?" Orochimaru asked, his yellow, vertical-slit eyes narrowing.
"As you can see, I've been betrayed by the village," Sakura sighed. "I'm currently locked up tight in Hōzuki Castle, so as long as I remain here, I won't be able to help your main body… if you catch my drift."
Leaving aside her reflexes and nervous system in general, the original Sakura's brain was quite capable, so to speak. She was intelligent and had an excellent memory… but she was no genius. She did the best she could with the modest abilities nature had granted her, yet she had never created any new medical techniques or ninjutsu. At best, she had just barely reached Tsunade's level by the end of Naruto's story… but went no further.
Following her transmigration, Sakura used her new body's intelligence to harness her creativity as a teenager from Earth, developing new techniques from the scientific knowledge she had acquired as a student… but if, for example, someone were to ask her to create a technique capable of resurrecting the dead, as Tobirama had pioneered, she would be completely stumped, for the Spiritual Plane and the Realm of Souls lay entirely beyond her comprehension.
"You wish to learn Sage Mode, then?" Orochimaru said, licking his lips. "After countless modifications, my physique grew strong enough to withstand nature energy, but unfortunately, unlike that fool Jiraiya, my body was never able to produce enough chakra to balance out the three energies."
In his youth, Orochimaru had attempted and failed the Snake Sage's trial.
The only way to train in Ryūchi Cave's Sage Mode was to have the Snake Sage bite and inject you with her Sage Power. The nature energy would rampage throughout your body, and the only way to complete the training would be to instinctively balance out the three energies… or die trying.
"I nearly died, and it was only thanks to my Orochimaru-Style Body Replacement Jutsu that I made it out alive," Orochimaru reminisced. "Unfortunately, since you do not currently have said Snake Sage on hand, I'm afraid you cannot learn Ryūchi Cave's Sage Mode. Not that that old snake would have accepted you, disciple of Shikkotsu Forest."
"Then, what do you know about Shikkotsu Forest's Sage Mode?" Sakura asked curiously.
Orochimaru shook his head.
"To my knowledge, no human has ever emerged whole— fully sound of mind, body, or soul— from the Great Slug's training since Tsunade stumbled upon Katsuyu's domain," he said. "I had discussed the matter with Tsunade before our… parting, but the only piece of information I gleaned from our conversation is this: the Slug Sage's ultimate technique is said to be the antipode of the Snake Sage's... and I don't know what that is, either, seeing as I failed to become a sage."
Sakura clicked her tongue in annoyance.
Back to square one.
"Not that you'd be able to mould Senjutsu in your current state," Orochimaru said dismissively. "Your chakra is impressive, but your physique is somewhat on the fragile side. As you are now, I'd compare you to a glass filled to the brim with water. Your vessel is insufficient, so adding any more water would cause it to rupture…"
"… so, I need to lift, basically," said Sakura grimacing. She then brightened up, remembering the technique scroll she had stolen from Dr Shinnō. "Oh, but I have that!"
"It's an excellent technique. I was the one who helped Shinnō steal Tsunade's Kinjutsu and Might Duy's Eight Gates training method in exchange for his assistance in developing my regeneration techniques, so I ought to know," said Orochimaru, nodding. "And after twenty years, I'm certain he must have polished his Body Revival Jutsu to even greater heights…"
Sakura's path was clear: she needed to get swole, and only once she was certain she wouldn't explode would she and Orochimaru's imprint develop a new jutsu that would help her cheat out Sage Mode.