Nana Shimura/Synopsis

History
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 was chosen to inherit One For All from the sixth user.

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 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 alongside 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 Tomura, 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.

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.

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 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.