Ichigo Kurosaki almost died in the latest episode of Bleach: Thousand Year Blood War. And in fact, he likely would have if not for a secret intervention by Aizen. How did Aizen save Ichigo and what exactly did he do?
Before the fight between Ichigo and Quincy leader Yhwach, Yhwach’s visited Aizen. Aizen was trapped and imprisoned within the Soul Society. Yhwach visited Aizen to ask for his help in defeating the Soul Society, and Aizen refuses. Rather than killing Aizen on his refusal, he lets him go as he doesn’t have time to annihilate him.
This encounter between Soul Society’s previous arch-enemy seemed minor, but it was actually very significant in the plot line of TYBW.

This is when Aizen secretly helps Ichigo. Although Aizen is imprisoned, he still has access to a very minuscule amount of power. He uses this power to slightly alter Yhwach’s senses and perception.
Fast-forward to Ichigo and Yhwach’s fight. It becomes clear that Yhwach far surpasses Ichigo in power. But there is one weakness that lets Ichigo escape. The Quincy’s are limited in the amount of time they can spend in the Soul Society before they must return to their home base. Once this time limit has expired they are unable to continue fighting and have to retreat.

This is how Aizen ultimately saved Ichigo. By altering Yhwach’s perception, Aizen bought Ichigo just enough time in the battle to exhaust the Quincy’s time limit and let Ichigo survive without being captured.
The details of how Aizen saved Ichigo was revealed when Yhwach realized what had happened and questioned his second in command, Jugram Haschwalth, for not informing him of the the subtle perception shift after he visited Aizen. Jugram goes on to let Yhwach know that even if he informed him of what Aizen did, he would have fought Ichigo anyway, so there was no point.
Why did Aizen save Ichigo and will he return again in TYBW? It appears he still has a part to play in Ichigo’s story. What that is, we have yet to find out.
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