At age 4, when Izuku picks up a notebook, it bonds to him, and on the first page, it tell him that should a person give permission, he can store a quirk in the book. It also says should he want to use a quirk, he must keep the book open to the quirk's page, and should he want to give up a quirk, he must get permission from the one taking it. Izuku and his mother notice that the book has become nicer looking and thicker with more pages :notebook_with_decorative_cover:
In school, he explains the quirk and is made fun of for it. :notebook_with_decorative_cover:
A year later, a girl in his school named Himiko Toga asks him to take her quirk. She wants to be a hero, and with the quirk she has, she never will. Izuku notices her fear, and insists that he and his mother meet with the Toga family before the transfer. :notebook_with_decorative_cover:
The Midoriyas and Togas meet and Himiko and Izuku become fast friends. Inko explains Izuku's quirk to the Togas and their relief is noticeable. Himiko tells Izuku that if she gives him the quirk, he needs to give her a quirk when he gets another in his book. :notebook_with_decorative_cover:
Izuku and the group discover several things about his quirk. The first is that when the person giving up the quirk consents but cannot read or write well, a blood related witness can sign for them. :notebook_with_decorative_cover:
The other is that the book takes all elements of the quirk, not just the parts that need to be activated. Himiko's sharp teeth become dull, which clues them in to this fact :notebook_with_decorative_cover:
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