Bleach's star is Ichigo Kurosaki, and for years on end, he had no idea that his spiritual powers ran in the family. He tried to keep his Soul Reaper career a secret from his father and his sisters, but in reality, Isshin had once been a Soul Reaper himself, and his mother Masaki had once been a pureblood Quincy archer.
Ordinarily, a Soul Reaper and a Quincy wouldn't even become allies in those days, let alone get married, but Isshin Shiba and Masaki Kurosaki met under unusual circumstances, and they had a common enemy to defeat -- a special Hollow that Sosuke Aizen had created as part of his plans. The story of how Ichigo's parents met was a bloody one.
When Masaki Kurosaki Sensed Danger One Night
During the Thousand-Year Blood War arc, Ichigo finally learned the truth of his parents' first meeting, which took place several years before Ichigo was born. At the time, Isshin was Captain of Squad 10. Isshin had only a passing interest in the world of the living, but he dutifully responded to a distress call from Naruki City, where he encountered a unique Hollow called White, Aizen's own creation. White was strong enough to face a Captain like Isshin, and nearby, a certain adolescent Quincy sensed the battle -- Masaki Kurosaki.
Ryuken Ishida's family had ordered Masaki to stay out of harm's way, since they couldn't afford to lose a pureblood like her, but that didn't stop her Masaki from rushing to the scene of battle, where Isshin and White were fighting. Masaki couldn't stand the thought of someone dying because she had failed to help, so she joined the fray with her bow and arrows, luring White into getting closer. At point-blank range, Masaki injured White with her arrow, and White self-destructed while biting her shoulder to infuse its essence into her. Masaki gained a positive first impression of Isshin, and they would meet again very soon under even more trying circumstances.
Isshin's Sacrifice & Masaki's Salvation
Masaki was poisoned with White's Hollow essence, and in time, it would kill her. When Masaki collapsed in the Ishida home, a desperate Ryuken Ishida took her outside, where he encountered Isshin Shiba, who had returned. Soon after that, Kisuke Urahara the rogue appeared as well, and he helped Ryuken and Isshin assess Masaki's condition. Kisuke helped Isshin transfer his Soul Reaper powers into Masaki to counteract White's powers, and in so doing, Isshin became an ordinary human -- a sacrifice he gladly made.
Masaki was stabilized, and years later after college, she met Isshin again. The two eventually married and started their family, while Ryuken Ishida kept his distance from his acquaintance Isshin for years on end. Ryuken was grateful for the help, but he didn't see Isshin as a dear friend. Notably, Isshin gave up the Shiba family life and adopted Masaki's family name instead.
This scene established a number of key elements in Bleach, including demonstrating Isshin's incredible willpower and his selfless nature -- something his son inherited years later. No ordinary Soul Reaper Captain would give up their power to save someone they barely knew, especially not a Quincy -- a traditional enemy of the Soul Reapers.
Moreover, White's transference into Masaski is what led to Ichigo's inner Hollow taking root, and White's essence would become Zangetsu, Ichigo's zanpakuto spirit. Thanks to Ichigo's thoroughly mixed heritage, he has a bit of everything inside him -- humanity, Soul Reaper powers, a Hollow and Quincy blood. Through Masaki, Ichigo was born as king Yhwach's "son," but in a tragic twist of fate, Masaki's death came about because Yhwach stole her powers, leaving her defenseless against Grand Fisher.
Now it's up to Ichigo to finish what his parents started, bringing peace and balance back to the world so everyone can live happily, from Soul Reapers and Quincy to humanity and beyond. That's his family's legacy, and he takes it seriously.