u/Sakumotosan

Think about how fragile the timeline of Bleach actually is. If Hisana had never met or married Byakuya Kuchiki, the entire series literally falls apart before it even begins:

Rukia dies as an orphan: Hisana’s dying wish as Byakuya’s noble wife was for him to find Rukia and adopt her into the Kuchiki clan. Without that wedding, Rukia grows up starving in the Inuzuri slums and never becomes a Soul Reaper.

Ichigo never gets his powers: If Rukia isn't a Soul Reaper assigned to Karakura Town, she isn't there when Fishbone D attacks the Kurosaki clinic. Without Rukia to jumpstart Ichigo’s dormant powers, Ichigo and his sisters likely die in first ep (or Isshin is forced to break his seal early under tragic circumstances).

Byakuya’s whole arc disappears: Byakuya's inner conflict during the Soul Society arc only existed because he swore on his parents' grave to never break the law again after breaking clan rules twice for Hisana and Rukia. Without them, he’s just a cold, ruthless noble.

Aizen’s scheme gets derailed: Aizen hid the Hogyoku inside Rukia precisely because of her position as a noble assigned under Urahara’s eye.

Hisana marrying Byakuya was the ultimate secret catalyst. Without that one relationship, Ichigo never meets Rukia, never learns Bankai, and Soul Society falls to Aizen and the Quincies.

What do you think—is Hisana's marriage the biggest butterfly effect in the entire series?