The dialogue in this section is going to be from the comic books themselves, so I cannot take credit for it.
Regarding the interactions between Splinter and Shredder, Splinter is the reincarnation of a medieval ninja who was slain by Shredder, who effectively became immortal later on.
"I am Karai, of the Foot Clan, and you are a prisoner. You would do well to remember that. Just as you would do well to show respect, rodent, and bow down before…" She gestured towards Shredder grandly, her eyes flintily worshipful towards her grandfather. "Master Shredder!"
Raphael felt a shiver run through him as all eyes turned in respect and fear towards their master, who rose above them dressed in steel, black and blood-red.
All eyes, that was, save for those of the rat, who seemed determinedly unimpressed by what he was seeing. Raphael had to admit that the rodent had guts — most people would be frightened out of their wits to be bound and helpless before a clan of ninja, but he seemed more… annoyed than anything else. Either he was an expert bluffer who hid his fear better than anyone Raphael had ever seen, or swaying him to join the Foot Clan would be more difficult than the turtle had thought it would be.
Karai, meanwhile, was regaling the rat with descriptions of the Foot."We are assassins. We are thieves. We are are ninja," she declared to the rat's face.
"I know what you are," he said quietly.
It was such a strange answer that Raphael blinked, and glanced briefly at Master Shredder. How could this rat mutant know about them, when they knew almost nothing about him? It wasn't as if the Foot Clan's operations were well-known outside of the underworld.
Karai smirked. "Do you?" she said sarcastically. She began pontificating again — something she loved to do, but rarely got the chance to do, in Raphael's experience. "Then, do you also know why we have brought you here?A war is coming, rodent—one that will begin in the filthy streets of this city and spread to engulf the entire world. A war for power and control—a war the Foot Clan has planned for ages. And with Master Shredder leading us… a war the Foot Clan will dominate."
Raphael knew nothing of this plan, but then, he didn't need to know, he thought a little bitterly. He was a sword, and a sword did not need to know the plans of his master.
The rat shifted slightly. "And what has this war to do with me?" he asked, still sounding unimpressed.
Either the rat was the gutsiest person that Raphael had ever seen, or he was too stupid to figure out what was clearly in store for him. He wasn't sure which it was, but it was definitely one of those two.
"You are living proof that our foes have begun enhancing their standing armies with mutated soldiers of remarkable abilities… as have we." Her gimlet-like eyes turned briefly towards the shadows where Raphael was standing.
"You have not answered my question—what does this have to do with me?"
"You have the opportunity to join us today, mutant. To become a soldier for the Foot Clan," Karai declared, in a tone that brooked no refusal. "But first… a test!"
Master Shredder made a gesture, and Karai sliced the ropes from the rat's wrists, then tossed her katana into his hands. He looked mildly perturbed by this, rather than alarmed.
And from the crowd of Foot ninja came the biggest of them — a behemoth at least seven feet tall and packed with dense slabs of muscle, and a kusarigama clutched in his hands. Though he was masked like all the others, he went bare-chested as a sign of his confidence in his own abilities.
Raphael tensed and leaned forwards, his eyes caught on the rat as the other mutant began leaping aside from the whirling chain being thrown after him. He was rooting for the rat on principle, and perhaps out of the selfish desire to see him win so that the Foot Clan would induct him into their ranks. So Raphael wouldn't be alone anymore.
But as the rat leaped and whirled, Raphael felt disbelief taking the place of his prior eagerness. The rat fought like a ninja already trained — he handled the katana as deftly as if he had been born to wield it, and easily drove back the massive ninja with a knee to the gut. The ninja was good, especially for someone as massive as he was, but the rat was clearly much better. Who had taught him how to fight like this?
Finally the rat seized the chain and swung around to kick the ninja in the face, knocking him to the floor. He raised the katana over his head, but did not lower it.
"Why do you hesitate, mutant? Kill or be killed!" Karai ordered.
The rat did lower his katana then… but not towards the fallen ninja. "No. I will not execute this man," he declared calmly.
Karai flew into a rage. "Fool! Master Shredder commands it! Do you not understand what is at stake should you disobey?!"
Raphael felt a heavy weight settle in his stomach. He knew perfectly well what was at stake — the mutant's very life. His fingers moved up to touch his unmarred cheek, and he hoped with all his might that the mutant didn't risk Shredder's displeasure by sparing the man.
"Yes, young lady, I fully understand what is at stake. Honor. My honor," he said, gesturing at the ninja around him. "I know much of the Foot Clan... Much more than you could possibly realize. Most important, I know that this was once an honorable clan—but no longer." He pointed at Master Shredder, and Raphael felt a shiver of horror go through him. "No, its honor was destroyed from within by another like you. He, too, desired power above all other things—complete domination at all costs. He, too, was wrong."
Karai nearly went berserk, shouting threats at the rat that Raphael barely paid attention to. His mind was whirling with questions about the bizarre things the rat kept saying — he spoke as if he had spent years around the Foot Clan, had seen it in action long ago and seen it evolve over time. But that wasn't possible — it couldn't be.
As far as Raphael knew, there were no mutants older than he was — at least, not much older. And he knew the Foot Clan from the very moment he had mutated. How could this rat know so much when he had never had the opportunity to learn about it? How could he know? Raphael's head spun, and he was almost tempted to step forward and demand to know where the rat had learned these things — how he could possibly know. Tempted, but not enough to break ranks.
"I would rather die than join you and lose my honor," the rat declared in a ringing voice. "You defile the Foot Clan. I will not be party to this disgrace."
Raphael steeled himself for what was inevitably coming next. Either Karai or Master Shredder would lash out and end the rat's life for his disrespect, and for his blatant refusal to join the Foot Clan. He waited for the flash of steel, the sharp tang of blood on the air.
Instead, Master Shredder murmured, "It… cannot be. Hamato Yoshi?"
Raphael's head snapped around. Did Master Shredder… know this rat?
The rat replied, "Oroku Saki?"
Master Shredder lifted his helmet and pulled the mask from the bottom half of his head, revealing a strong-boned face marred by a scar running over one eye.
His voice rang out through the echoing room. "I would ask how it is you are here, Hamato Yoshi, and in this form, but I have not returned from my long death to waste precious time on the inconsequential. Karai, destroy him."