The Hakutaku form of Kamishirasawa Keine.
The nickname comes from the fact that Keine is the Extra Stage mid-boss (or "EX mid-boss" for short) of Imperishable Night in this form.
As a joke, fans commonly say "Caved by Ex-Keine."
Person: red eyes, long hair, two-tone hair, white, green hair, sidelocks, bangs, horns
Clothing: green dress, collared dress, puffy short sleeves, red neckerchief, blue skirt, white legwear, bobby socks, black footwear, shoes
Accessories: footwear bow, horn bow, red bow