At some point in the past, En received One For All from his predecessor, the fifth user Daigoro Banjo, after he was injured and trapped under a pile of rubble. Daigoro extended his injured hand at him so En could acquire One For All after consuming Daigoro's blood.

Sometime later, All For One attempted to steal One For All from En, but failed. At some point, as the sixth user, he met Nana Shimura. After being fatally wounded in a battle with All For One, En gave Nana some of his hair so that she could become the seventh heir of the One For All Quirk.

(Source: My Hero Academia Wiki)