It was about two years after Kazui was born when Ichigo realized that he couldn't recall the all the details of her smile anymore.
Which is strange, because he has been so used to retracing - in his mind - the specific curves of her lips and the particular way her pupils caught light. He has been doing that for many years. He has had to do that for many years. Even before the fall of Yhwach, even before the fall of Aizen, even before the image of her faded into thin air before his eyes after he lost his shinigami powers.
Ever since that first time she told him that she will stay in her world, and he will go back to his.
He reacted by telling himself: "Now I remembered why I wanted to save you so much, Rukia." He said remembered, but it was more of an act of unwilling admission. He was so desperate, so completely destroyed when she was taken away by Byakuya and Renji because her life was in danger, first and foremost; and because he failed to "protect that one person" the way his name promised, once again. But some core, instinctive part of him just desperately wanted her back.
He had no reason to assume that, but somehow when he screamed "to my soul" as he achieved bankai for her, when he descended in front of her and said "I came to save you", and when he smugly smiled while standing between her and the SÅkyoku...he thought that after all of this is over, life will go on as it did before her arrest, with her by his side, forever.
And when he found her by the Shiba residence, in her purple yukata, standing in a field of wild flowers, dark hair flowing in the wind, he realized that it was never going to happen. She was already out of his reach, she was always out of his reach.