Warning! Spoilers for My Hero Academia Chapter 310 ahead!
Fans of the My Hero Academia manga have long awaited the introduction of the first to wield One for All, and at last that time has come in Chapter 310. Now that an unconscious Midoriya has recently communicated with the remnants of One for All's previous users directly for the first time, more about them, and their powers, are finally being revealed. Yet, even after that significant turn of events, the first user had remained largely mysterious.
Thus far in the story, little has been known about the first user, aside from the fact that he was once All for One's brother, and that he seemingly didn't have a quirk at birth. It's also clear that he and his brother didn't get along, and the lineage of One for All's users was established out of his desire to rebel against his brother's evil actions.
In Chapter 310, an aside is shown of the vestiges of previous users discussing the situation regarding Tomura Shigaraki, and Midoriya's newfound desire to save him rather than defeat him. The first user, now identified by name as Yoichi, addresses two other users who had previously held back in cooperating. One of these is the second user, who is revealed to have rescued Yoichi from his brother's control and first resisted All for One. Yoichi draws a comparison in the story of the past to Midoriya's feelings towards Tomura now, which has the desired effect on the second.
Yoichi seems to have an inherent kindness, as well as a strong sense of what's right and wrong and a moral obligation to correct it. He doesn't see the world in black and white, however, which makes him an excellent starting point for the quirk. Since the only thing Yoichi had to offer was the heritability of his quirk, this moral compass function still allows him to bring something to the table for future users. If gaining advice from the previous vestiges is a constant ability of One for All, Yoichi would essentially function as a conscience, advising future users on difficult moral issues. With later users bringing along a new ability or experience as a pro hero, Yoichi's importance might diminish without this influence.
With the reveal of Yoichi's name and the new aspects of the origin of One for All, there are only two users who have yet to be properly identified: the second and the third, who initially rejected Midoriya as a successor and whose names and quirks are unknown. The second and the third knew each other, and were both part of the resistance against All for One that rescued Yoichi in the first place. It is known that nearly all users died as a result of All for One's actions, and that everyone before All Might and Deku had a quirk from birth. Since Yoichi's opinion seems to have swayed the second and third users to support Midoriya, the time might finally come for Midoriya to wield all six of the abilities stored in One for All, finally bringing the quirk to its full, true power just in time for the final confrontation in upcoming chapters of My Hero Academia.