Ukitake Jyuushiro had known Byakuya since his birth. Children were rare in noble families and the birth of a new heir, especially in the Kuchiki, was a momentous occasion. Jyuushiro was one of many in a long line of official visitors who attended the ceremony where Byakuya's father formally presented the infant; his sole distinction being that he was the only one, father included, who knew how to change Byakuya's diaper when he shat himself. After all, you don't raise five brothers and two sisters without learning a thing or two.
The next time Byakuya encountered Jyuushiro, it was as the man's unwilling student. His father had determined that the boy was too advanced for his current kendo instructors. Byakuya wanted to train with his father, but was stuck with the Jyuushiro instead. Bitterly upset at receiving what he perceived as inferior instruction, Byakuya promptly informed the sickly captain that there was no way Jyuushiro's sword could ever match the newly-manifested Senbonzakura and that he should go home before he hurt himself. Jyuushiro smiled and replied that it was a good thing he had two swords.
As a young boy struggling to learn shunpo under the tyrannical auspices of Yoruichi, Byakuya sometimes vented his frustrations at his constant humiliations by stealing peaches from the Ukitake garden. He did this less for the peaches than for being able to say he'd outran the legendary captain as Jyuushiro would emerge from his house, huffing and puffing, to chase the young prodigy. When Captain Ukitake later met the newlywed and very distressed Kuchiki Hisana at a tea party, she confided to him she felt horrible because she had no way of getting Byakuya to take care himself. She was too meek to make him go to bed when he was overtired, go to 4th for his wounds or restrain his temper when the family insulted her. Jyuushiro gently squeezed Hisana's hand and told her that he was confident she would find a way, "but if all else fails, make him think he's winning by doing what you want him to do." Then he asked her if she wanted a peach.
When Hisana died, Jyuushiro was the only person to hug Byakuya. It wasn't until a half-century later when he reconciled with his adopted sister that Byakuya changed this.
As a young teen, Byakuya had great difficulty in finding additional information about the "weird feelings" he got around girls and why he might suddenly wake up one day decide they looked better naked than they did with clothes on. Upon finding both his father and the servants mysteriously absent, he turned to his kendo instructor. Jyuushiro magically turned an even lighter shade of pale, but nevertheless invited the young lord to sit down as he casually poured himself some hard tea he kept hidden from Shunsui's greedy lips.
It was a very informative day for Byakuya.
Perhaps a little too informative.
Once Byakuya finally realized the depth of his feelings for Hisana, he remained in frozen inaction: too bewildered by the depth and passion of his feelings, too shaken from the idea of going against the foundations of his life and the orderly goals that had been laid out for him since birth. With Hisana too accepting and timid to fight for the gift fate had dropped into her guilt stricken hands, it was left to Jyuushiro to strong arm his former student into marriage. Byakuya's almost silent, "You were right," upon receiving his bride from Jyuushiro's arm was the sincerest expression of gratitude anyone had given the old captain.
Following Hisana's death, several family members convinced the newly widowed Byakuya that Jyuushiro's frequent visits to his late wife were a means of spreading his illness to her. Despite his turmoil, Byakuya entered the 13th division by stealth at night and impaled Jyuushiro through his own sleeping mat, pinning him to the ground. Old ties die hard, however, and Byakuya was desperate to hear his former teacher declare himself guilty with his own lips: Byakuya missed the soul chain. Still alive, but dying by the second, Jyuushiro desperately explained the truth he'd only recently discovered: the Kuchiki had betrayed him. His brother and those allied with him were afraid Hisana would have a child. Though Byakuya's legendary speed saved Jyuushiro's life, Byakuya's was now destroyed. The one who had been his whole world slain and his hands stained with innocent blood, Byakuya vowed to slaughter his clan before killing himself. The massacre was averted when Jyuushiro used Byakuya's guilt and the oath he swore on his father's deathbed to restrain him. The two did not speak again as anything more than colleges for decades afterwards. Each considered this time among their deepest failures: for Byakuya, it was the night he utterly destroyed his honor; for Jyuushiro, it was his hesitance that killed Byakuya's smile.
After an ill Jyuushiro angrily confronted Byakuya over his support of Rukia's execution, Byakuya knew that his old teacher would attempt to save his adopted sister. As Sebonzakura shattered in his hands amidst the ashes of the sogoukyu, Byakuya decided that he was content.
Byakuya's love of burying everything he ate in wasabi was well known by Jyuushiro. The senior captain often tried to see if he could find a dish that was too spicy for the Kuchiki nobleman, but each attempt was a spectacular failure. When even Kukaku failed to produce something sufficiently corrosive enough to defeat Byakuya's iron palate, Jyuushiro gave up all hope of ever conquering his former student…that is, until he met Sado Yasutora and discovered that the fire of chili peppers operates on a completely different principle than wasabi. When a desperate Byakuya was forced into using Renji's beer bong because of an artfully placed insanity pepper, Jyuushiro had his camera ready.
The high wall that had been between Byakuya and Jyuushiro finally collapsed when Byakuya returned from Hueco Mundo to find Jyuushiro standing alone, gaunt and ragged, bearing a request: will you be vice-captain? Byakuya knew then question that really lay before him: did he decline and fight on alone, uncaring and invulnerable to all pain, or did he again return to his old mentor; to watch as he inevitably drove himself to death (so much like Hisana) and was slain, leaving Byakuya with the crushing responsibility of Captain-General in a losing war? In the end, there was really only one choice.