Nana Shimura/Synopsis

History
Like most people, Nana Shimura 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 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 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, Kotaro Shimura, with whom she had an excellent relationship. At some point, 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.

To protect Kotaro from All for One, Nana even had to cut off anything connecting her to her son, to the point she even doctored the family register, so Kotaro can't be tracked back to her. Separating from her son was a heavy emotional blow for both of them, as she was forced to declare that she "has no son". Nana left a letter to Kotaro explaining the reason for this, but Kotaro grew bitter by this abandonment, despising anything related to heroes and a certain grudge towards his mother.

Unfortunately, all this would end up backfiring. His son, Tenko Shimura, would end up accidentally killing his family by manifesting his Quirk for the first time. And since both Gran Torino and Toshinori kept their promise to her, Tenko eventually became a villain under the tutelage of All For One, now under the name of Tomura Shigaraki.

Prior to Tenko's birth, during Toshinori's final year of high school, he, Nana, and Gran Torino faced off against All For One in a devastating battle. Realizing they wouldn't be able to win, Nana pushed Toshinori away, who was forced away from the battlefield, despite his crying and insistence. Nana points toward her pupil, smiling and telling him that it is his turn and entrusting his training to Gran Torino, before turning back to face All For One alone. The villain thanks her for the "comedy", killing her in a giant explosion. At some point later, All For One got ahold of one of her hands, and gave it to her grandson Tomura to replace one of his father's hands that he accidentally destroyed.

U.A. Sports Festival Arc
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.

Pro Hero Arc
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.

My Hero Academia: Heroes: Rising
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.

Paranormal Liberation War Arc
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 Master's darkness, embracing the path of villainy. During his fight with Tomura, Izuku manifests Nana's Quirk, Float, and uses it to contain Tomura in the sky while pulling his allies to safety using Blackwhip.

Unfortunately, during the course of the fight, the vestige of All For One manages to gain control of Tomura's body and attempts to steal One For All from Izuku, but something unexpected happens and everyone is transported into the world of vestiges. As Izuku struggles to move, he sees that All For One and Tomura are fused together, and they fight to be in control.

Suddenly Nana appears beside Izuku and tells him "they" will help him. She then speaks to All For One and expresses her disgust at seeing him again even after death. All For One is amused with Nana's appearance, taunting her as he explains to Tomura that she is his grandmother.

All For One explains to Nana as well as Izuku and Tomura the reason why the vestiges are present in the users of All For One and One For All. When finished, he "invites" Nana and Tomura to greet each other. Nana recognizes him as the child of her son, but Tomura replies that it doesn't matter. Nana somberly listens as Tomura admits that no matter how they ended up in this situation, he still hates her and begins eroding the Vestige world away.

In order to protect Izuku, Nana resists Tomura's crumbling of One For All, as All For One gloats Tomura's power born of his hatred, even when he himself is affected as well, but considers that it is still not enough when he suddenly sees that Nana is accompanied by his little brother and the first user, Yoichi Shigaraki.

Between the two of them, Nana and Yoichi start to reject Tomura's power, so All For One decides to use his in combination with Tomura's to seize One For All. The villain derides Izuku, claiming to his brother he made a mistake in selecting him. Yoichi refutes his big brother's words, telling him that Izuku is the most suitable heir for his Quirk, and together with Nana they reject All For One, 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 to him about the true nature of One For All. After this discussion, she approaches him and asks if he can kill her grandson, Tomura. Nana clarifies that she is only testing Izuku's resolve, as 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 is aware that he might have to kill him, believing it to be possible for him to be too far gone. However, in their vestige link-up, Izuku had sensed a "crying child" deep within Tomura, and even if he has no idea how he will accomplish it, he wants to be able to understand that side of him, and if he can, save him. Nana starts to cry, agreeing to support Izuku and thanking him, saying as well to send her regards to Gran Torino.

Tartarus Escapees Arc
When Izuku is starting to tire out after days of fighting villains nonstop, Nana and the other inheritors of One For All try to convince him to rest before Danger Sense warns him of another threat and heads off, ignoring their words. In the vestige world, Nana fears that her emotions are getting the better of Izuku, while the other inheritors assure her that by going over his limit, he's acting like a true hero would do.

Final War Arc
As the final battle between the heroes and villains commence, Izuku prepares for his final encounter with Tomura. While a snag in the plan delays his arrival, he eventually makes his way over to the U.A. Flying Fortress designed to contain Tomura to fight him. After regaining his bearings, Izuku confronts Tomura, who over the course of the battle has transformed himself into a hulking beast covered in fingers, and addresses him as "All For One", asking if Tomura is still within him.

"Tomura" claims that he no longer exists, as he and All For One's psyches have melded together into a perfect new being. However, Mirio Togata reveals that during their battle, the villain reacted to one of his taunts like that of a child, believing them not to be as melded as he thinks. The spirit of Nana looks towards Tomura's left hand, which is comprised of the faces of the deceased Shimura family, including that of her son, Kotaro. Seeing him look at her, she tells Izuku that he is still in there, sparking the battle to resume.

Izuku activates the Quirk of the Second User, Gearshift, and now with all the Quirks unlocked, unleashes the full power of One For All upon Tomura, overpowering him, with Nana and the other predecessors right there behind him.