Ichigo's bond with Zangetsu, Hollow Ichigo, and Muramasa. Best exemplified with this line, where Ichigo highlights the greatest difference between him and Aizen.
"In the end, I chose not to be alone."
Muramasa is the one that reminds Ichigo that You Are Not Alone. And for giving him his power, all he ever asks of his new wielder is that he never let him go.
Muramasa: "Is there now any part of you that hates me?"
Ichigo: "No."
Muramasa: "Then I am rewarded."
Zangetsu repeatedly warns Ichigo that Nelliel won't help them because they have never met in this timeline. However, Nelliel, who has become an adult permanently, shows up to save the group from Tosen.
