Like most people, Nana was born with a Quirk of her own, Float. While it is unknown how the two heroes met, she was a close friend of Pro Hero Gran Torino. At some point in her life, Nana received One For All from the sixth user, En just before his death at the hands of All For One.
Shortly after inheriting the One For All, Nana had a strange dream, where she encountered one of the previous users shrouded in dark fog telling her "It's not time yet…". Nana was mystified and decided to tell her friend Gran Torino about this strange experience.
Eventually, she met Toshinori Yagi, a boy who despite being Quirkless wanted to create a world where everyone can smile and live happily together. For that to happen, the world needed a "Symbol of Peace" to inspire a new age of harmony. Toshinori believed he could become the pillar that inspires people everywhere. Nana liked Toshinori and his strong sense of justice, and decided to become his mentor. Entrusting him with the One For All, Nana took it upon herself to train All Might and she was assisted by Gran Torino.
During her lifetime, Nana married and had a son. Soon after, her husband was killed. To spare her son from the same fate, Nana left her son in the care of a foster family, telling Gran Torino and Toshinori to never come into contact with him under any circumstance. However, it was through keeping this promise that Tenko, her grandson, eventually became a villain under the tutelage of All For One.
Eventually, Nana was killed by All For One in a fight sometime after passing along One For All, entrusting Toshinori's training to Gran Torino. In All For One's fight with All Might, he describes the way she died as "pathetic." One of her hands was given to her grandson Tomura to replace one of his fathers hands that he accidentally destroyed.
During the fight between Izuku against Hitoshi in the tournament of the U.A. Sports Festival, Izuku is brainwashed by Hitoshi's Quirk, who orders him to walk out of bounds. Although Izuku tries to resist, he cannot disobey the order, but when he is about to cross the edge of the ring, Nana and the rest of the previous users appear to Izuku as shadowed mental figures. They uncloud his mind and allow Izuku to use One For All in his fingers, allowing him to escape the trance and win the fight.
After Izuku Midoriya falls asleep after an exhausting training session, he experiences the vestiges of One For All. Nana Shimura and the other users of One For All appear in his dream. While Izuku doesn't recognize her, he later identifies her as All Might's master. After the dream concludes, she is seen with the other predecessors blessing Izuku.
The next day, Izuku narrates his dream to All Might. All Might remembers what Nana had told him about the vestiges of One For All. She had speculated that the Quirk stores the will and memories of the previous users in order to activate its power. She had found it romantic that even after her death, all One For All users will meet each other.
During a devastating battle between a villain with a copy of All For One's Quirk, in a last-ditch effort to defeat him, Izuku transfers One For All to Katsuki Bakugo, even at the cost of rendering himself Quirkless once more. Through the dual power of the One For All's, the two manage to take the villain down, even as the remaining embers of One For All seemingly disappear from Izuku. However, Nana and the other users of One For All, wanting Izuku to continue to harness the power, and recognizing his heroism of being willing to sacrifice his power to save everyone, only temporarily transferred the power over to Katsuki, while the full essence of One For All remains within Izuku. Toshinori tearfully thanks his master and predecessors for staying with Izuku.
Tomura Shigaraki voluntarily undergoes a complex and painful four-month operation carried out by Kyudai Garaki to inherit the All For One Quirk. However, the heroes storm Dr. Garaki's labs by interrupting the process with weeks left to complete, and destroying the capsule containing Tomura.
Since his heart is not beating, Tomura is considered dead. However, mentally he is in a kind of mind world, where a shadowy version of All For One appears and tells him to come to him. The memories of Tomura's dead family members appear, all desperately trying to stop him from following All For One's footsteps. The last one to appear is Nana Shimura, who grabs the back of his head and telling him to not forget them.
Tomura destroys them all without hesitation, telling them not to reject who he is. The crumbling face of Nana watches her grandson walks into his Sensei's darkness, embracing the path of villainy.
During his fight with Tomura, Deku manifests Nana's Quirk Float and uses it to contain Tomura in the sky. Later, when Tomura attempts to steal One For All from Deku, the two are transported into the world of vestiges. As Deku struggles to move, Nana Shimura appears beside him and tells him "they" will help him. All For One is amused with Nana's appearance, taunting her as he explains to Tomura that she is his grandmother. Nana somberly listens as Tomura admits that no matter how they ended up in this situation, he hates her. Nana tries to resist Tomura's crumbling One For All, but is then accompanied by All For One's brother and the first user, Yoichi Shigaraki. Together, they push back Tomura's decay, resulting in Izuku and Tomura falling away from each other in the real world, knocking both of them out and stopping Tomura from stealing One For All.
In the aftermath of the battle, while Izuku is in a comatose state, Nana and the other predecessors speak with Izuku and she asks him if he can kill her grandson, Tomura Shigaraki. Nana clarifies that she is only testing Izuku's resolve. She is ashamed that Izuku is forced to take responsibility for her failures, but asks him that if Tomura is beyond saving, will Izuku kill him? Izuku replies that he wants to understand Tomura's background and try to save him. He does admit that if all else fails, he will kill Tomura, although at the moment he has no idea how he will accomplish this. Nana starts crying and agrees to support Izuku. Before Izuku leaves, Nana tells him to send her regards to Gran Torino.
When Izuku is starting to tire out after days of fighting villains nonstop, Nana and the other inheritors of One For All tried to convince him to rest before Danger Sense warned the latter of another threat and head off. In the vestige world, Nana fears that her emotions are getting the better of Izuku, the other inheritors assures her that by going over his limit, he's acting like a true hero would do.