In Hawk Moth's lair...
"I feel... the sadness of a girl who never gets to see her older sister again after their powers are lost forever. Such an easy prey for my akuma," his window opens to create a supervillain.
A single butterfly lands on the palm of his hand and he covers it with his other hand to fill it with dark energy, converting it into an akuma.
"Perhaps one of my akumas can ease her sad heart."
The akuma flies through Yusa and possesses her sister's letter, causing a moth-shaped emblem to appear on her face.
"Hello there, Yusa," Hawk Moth spoke through her. "Don't worry about me. I'm your friend. I feel the loss of your older sister, Misa Kurobane, and all this time you weren't aware of her spirit possessing your body due to your uncontrollably power. Worry not. With my power, you will retain your power, as well as your sister, and you will have control over it. Is that okay with you?"
"Yes, it is okay with me, Hawk Moth," Yusa grinned.
"Good, then you shall known as Medium."
With her agreeing to the deal, purple smoke covers her body and once that's done, she appears with pale skin, and her school uniform is in dark form.