In the beginning his change was unnoticeable.
Sure, they all saw how ridiculously powerful he had become, in his shinigami form with his zanpakuto in action. They saw all of that, but what happened in Soul Society, later in Hueco Mundo, and then back in Soul Society for the December War was more like a dream than anything else. When they returned to Earth life went on exactly as it did before.
Well, almost anyway.
Life and death situations have a way of changing people's life, forging some bonds, and changing others in ways that could not be disregarded. When the four returned to Earth after the world regained most of its former stability they were inseparable. Keigo, Mizuiro, and Tatsuki were welcomed into their group, but their relationship with them simply wasn't the same as their relationship with each other. It couldn't be helped. They were not part of their adventure.
Rukia, of course, was considered part of their group, but she was more or less a part time affiliate, an honorary member that appears periodically. The December War united as many lives as it took. Many previously filled positions became vacant and with the bankai she gained in the war (in addition to Bayakuya's blessing) she became the vice-captain of the thirteenth division. Her work and responsibilities kept her away from Earth most of the time but whenever she could spare a moment she would visit them.
Undeniably, most of her visits were made to him alone. Between those two was a special bond no one could comprehend fully, even themselves. Their meetings were always loud, both too stubborn to agree on anything, both too proud to admit they were wrong. She said she did not care for him and in turn he said he did not care for her. Yet, though at times she had legitimate reasons for being in the living world, more often, too often perhaps, she had to conceived creative excuses. Similarly, Ichigo developed an odd habit of leaving his window open even in the coldest weather. When he went on overnight trips he always leave a note, just in case.
At least once a month - more frequently during summer - there would be a group reunion. During those, she would appear in a gigai so the ordinary masses would not find the group talking to thin air. When they first reintroduced her to their acquaintances, they called her an old friend that had moved away. It was technically not a lie. Although Keigo and Mizuiro had little memories of her (as their initial meeting was wiped and their second meeting was relatively short), it didn't take long for them to spread the words that Ichigo had gotten himself a girlfriend in school. It was inevitable. From afar those two looked too much like a bickering couple married for way too long.
Ichigo's reaction to the rumors was quite similar to his reaction before – violent denial. Rukia's reaction was a perfect act of false obliviousness.
Time flew. With their high school life over, the first ones to move out of the country were Orihime and Ishida. By pure coincidence (or maybe not so much) they received scholarships to Harvard. By then teasing Ichigo and Rukia had long gotten old, and Keigo had turned his attention to Orihime and Ishida.
Ishida's reaction to the teasing was to blush and look away. Orihime's reaction was genuine obliviousness.
The first to notice the change was Orihime. In hindsight, this should not be a surprise, Orihime always had sharp instinct. In their final gathering before university started she flipped through an old photo album and made her innocent comment: "Kurosaki-kun, you look just like you did when you were fifteen."
Ishida looked past her shoulder at the album and agreed, "She's right, Kurosaki, you haven't changed at all."
And that was it. They moved on to a different topic without a second thought and the conversation sank to the bottom of their subconsciousness.
It was hard to say exactly when he realized. Perhaps it was when Keigo complained about getting his first white hair; perhaps it was when doctors and nurses at the hospital did not believe him when he first told them he was in the hospital for his residency; perhaps it was when he realized his sisters had blossomed into beautiful women during their high school graduation ceremony. It didn't really matter. What was important was that one day he realized just how true Orihime's comment was. "Rukia, call me crazy if you will, but I don't think I have aged since I was fifteen."
For once, Rukia did not argue.
They traveled to Soul Society to visit the one person they thought may know the answer. Urahara, no longer in exile after the December War, welcomed them to his office in his usual cheeky ways. He instantly sobered when they explained the purpose of their visit.
"Didn't I tell you there would be a price to pay for saving your friends, Ichigo?" he said eventually, catching Ichigo's eyes as he put down his pen. "The body is an extension of the soul, and as you already know a shinigami's soul never grows old because it is no longer bound by life."
"You are telling me that I will never age?"
"No, Ichigo, I am telling you that, technically, you are already dead."
A/N: I have always thought there must be a consequence to breaking your chain.
It's Christmas so be generous and review.