[Well, luckily for the two of them Katsuki isn't stupid.
The moment he sees the flame escape the boy's mouth, singing the fur on his ear and a little bit of the skin underneath, he lets go and jumps back a few feet, standing up on his hind legs with his arms in front of himself. He activates his Quirk, which causes his muscles to tighten and thicken, his hands to grow thick and sharp claws along with more fur up his arms, his teeth to grow into sharp fangs and his red eyes glowing yellow.]
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The moment he sees the flame escape the boy's mouth, singing the fur on his ear and a little bit of the skin underneath, he lets go and jumps back a few feet, standing up on his hind legs with his arms in front of himself. He activates his Quirk, which causes his muscles to tighten and thicken, his hands to grow thick and sharp claws along with more fur up his arms, his teeth to grow into sharp fangs and his red eyes glowing yellow.]
Grrr... don't fire! I'll kill you!