Nana Shimura/Synopsis

History
Like most people, Nana was born with a Quirk of her own. While it is unknown how the two heroes met, she was a close friend with pro hero; Gran Torino. At some point in her life, Nana was chosen to inherit One For All from the sixth user. She eventually met Toshinori, and became his hero mentor even though he was Quirkless. Entrusting him with her Quirk; One For All, Nana takes it upon herself to train All Might.

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. It is through keeping this promise that Shigaraki, her grandson, eventually becomes 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".

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 about his dream to All Might. All Might remembers what Nana Shimura 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.