If there was one thing that Kenpachi believed in, it was that he was invincible. It was not mere arrogance either. He was strong. Not even a handful of people lived to tell the tale of his prowess. Enemy and ally alike, he strived to beat them all; to be the best among them. Beating them was proof of his invincibility. To be the strongest. Incomparable to others. That was his goal. Even mighty Death Gods, with their flowery bankai could not stand up against him. He was truly the epitome of strength in his era.
And then, Unohana showed up. Bit by bit, with her smooth words, healing hands, and kind heart, she broke his impenetrable defense. With words which brushed ever so softly against his greatest attacks, like butterfly kisses, he saw her breaking what others could not. Suddenly, his beliefs were crushed under the hands of that gentle woman; and he found that he didn't mind it so much.
Invincibility was impossible. Born from the ideas of haughty men who eventually, due to said haughtiness, fell to their own demise. Sure, there were those who seemed indestructible. Strong beyond their own good, nothing could destroy them. But in the end, there will always be someone stronger than them. Defeated, all the airy views of themselves shattered under their opponents foul swoop. With that in mind, Retsu always made sure her feet were planted firmly on the ground of level-headedness. Because she knew, even if she could do things others could not, even if she could heal pains and sicknesses others can't, there were still things which were out of her ability. She could not regenerate limbs; she couldn't heal mental wounds; she could not bring back those who have passed on to the next life. So for her, invincibility was far from attainable.
When she first met with Zaraki, she thought he was nothing short of an idiot. She was pleasant, polite in fact, as she was with everyone. But he was so sure of himself, positive that he was unrivalled; Invincible. Behind a feigned smile, she predicted his demise. No one was invincible, and she pacified herself with this thought. But time and time again, he would prove her wrong. Just as she thought he had fallen too far into his own power hungry trap, he would hoist himself out of the pit and throw the damn fool who had even thought about taking away his title in, adding another frame to his victories. Over and over again, again he would defy her belief. Break it into half and step on it until it was nothing more than dust. Despite whatever foundation which she had laid her life principles on being shattered under Zaraki's feet, Retsu felt stronger and more confident than she had ever recalled herself to be.