Previous Successor Quirks

Previous Successor Quirks – Notes for Young Midoriya is a notebook written by Toshinori Yagi with information of several of the previous users of One For All.

Overview
As a result of the learning that One For All can assimilate the Quirks of its users and will manifest them at any time, Toshinori conducts a thorough investigation to find out as much possible about the previous One For All inheritors and their Quirks in order to help his disciple Izuku Midoriya when a new Quirk manifests.

Toshinori collects as many details as he can possibly get of the predecessors: their hero names and real ones, characteristics and abilities of their Quirks, details of their life, etc... even how and when they died. Toshinori writes all this information in a Campos notebook, as does Izuku with his Hero Analysis.

From the information obtained, Toshinori finds out that neither of the predecessors had peculiarly powerful Quirk, as All For One hunted down anyone with strong abilities. Also, the concept of a "One for All successor" was not implemented until Nana Shimura chose him. Until then, the heirs fought All For One and got themselves killed for it at relatively young ages. They simply entrusted the power to a closest ally to keep fighting, until the Quirk grew powerful enough to defeat All for One.

Despite his best efforts, Toshinori was unable to find any information about the 2nd and 3rd users due to the era they existed in and the multifaceted nature of the Quirks. Similarly, when he discovered that Hikage Shinomori, the fourth user, died at the age of 40, All Might scribbled out the cause of death due to suspicions he had about One For All's side effects.