It was not until his grandfather's funeral that Byakuya truly understood that Yoruichi was gone from Soul Society.
Truly gone – some little unacknowledged part of his heart had long suspected that she and Urahara and Tessai were simply hiding somewhere, cloaking their riatsu and giggling at everyone who had the nerve to get on with their lives.
He spent the entire process staring down at the ground, just waiting for that innocuous black cat to come slinking up under the seats and twine its way around his ankles. He knew exactly what he would do, too: he was going to sniff and train his eyes ahead, focusing on the pomp and ceremony that his grandfather was so fond of and resolutely ignore the demon trying to distract him, even as he watched her out of the corner of his eye and refused to kick her away.
Then the ceremony was over and he realized that she hadn't come. She wouldn't even return to even pay respects to his grandfather – his grandfather, who had helped train her and helped her make the transition to captain – and for the first time in all the years since he'd last seen Shihouin Yoruichi he got angry.
She'd one-upped him again.
They'd spent years in competition against one another, and the thought that he wasn't going to get a chance to truly beat her made something inside of him well up, something deep and frustrating that choked all the protests from the insulted child howling to chase after her.
He was not a child anymore: there would be no more chases. The next morning when he took up his blade, he did not imagine her on the other end of it taunting him once.