Byakuya had an incurable undying illness, which means he couldn't die. Gin, his best friend, made a promise that he would take care of Byakuya until death and the promise had been kept for 16 generations. However, Gin the Sixteenth had no intention of making babies for the next generation; instead, he kept his promise with Byakuya in a way that none of the previous Gins had done.