Hana Kurusu began as a relatively minor character in Jujutsu Kaisen, first shown descending upon an injured Megumi Fushiguro like an angel. However, over time, she's become one of the most important characters of the current arc. Hana is a vessel for an entity like Sukuna who calls themselves The Angel, and her cursed technique is the only known thing that could help Fushiguro and Itadori break Gojo out of the Prison Realm.
As a result, the sorcerers who joined the Culling Game sought Hana out in the hope that she could assist them. Surprisingly, the young girl came to Fushiguro and took him somewhere safe after he collapsed after his lengthy brawl with Reggie Star. Given the nature of the games, Hana's thoughtful action perplexed fans, as he was saved in a context that favors and rewards killing. Chapter 210 finally sheds some light on this, revealing that the two had previously met and providing a reason for Hana's selfless actions.
Hana's Life Was Unknowingly Saved By Fushiguro
The chapter began with a flashback to Hana's childhood, showing her alongside two other young children being fed inedible scraps by a cursed spirit. The children were held captive by this curse, which seemed to have some sort of maternal affection toward Hana and the others. They all knew the curse wasn't their mother, but anyone who was frightened of the monster, cried, or complained would suddenly disappear, so escape wasn't an option.
Before she was able to escape the curse, Hana was imprisoned by it for about a year. When she returned to civilization, she followed a dog with a white fluffy tail, and people rushed to find out where she had come from and to get her food and water. Hana, however, disregarded the advice and persisted in trailing the dog until it eventually turned around and started walking next to its owner. The child was directed to a young Megumi Fushiguro walking home with Gojo by the white dog, one of her Demon Dog shikigami. Hana was most likely given a second chance at life after being picked up by an adult from this point on.
Hana Had Hoped that She Would Run Into Fushiguro Again
Hana started to believe that she and Megumi would eventually cross paths after seeing Fushiguro and Gojo leave. She has a strong belief in fate, which Fushiguro's character frequently represents, so she had no doubt that she would run into him again someday. As a result, she referred to him as her "fated one" when the sorcerer awoke after being saved by The Angel's vessel. This greeting didn't make much sense before, but it is now obvious that Hana was recalling her time with Megumi in the past.
After being saved by Fushiguro, Hana began using her power for good, working hard to make sure that when she and the young boy who helped her reunited, she would be worthy of standing by his side. Fushiguro doesn't see himself as a good person and believes that he saves people on a case-by-case, selfish basis, so wouldn't agree with Hana's opinion of him. However, at his core, he is a good sorcerer who uses his power to help people. He saved Itadori's life without a second thought, and Hana's story proves that he was doing these selfless deeds even as a child.
Hana saved Fushiguro as a way of thanking him for doing the same for her. While they only met in passing, fate tied the two together, and now Hana is the only thing standing between Fushiguro and Itadori helping Gojo. While the Angel doesn't want to help the pair without them destroying the "Disgraced One" first, the entity and Hana seem to work as a cohesive unit, so her history with Fushiguro may be enough to override this decision.