The default costume of Nijisanji's Suo Sango, as seen in post #4040979.
She is portrayed with the same Selene Girls school uniform much like her gen-mates but set in red like Asahina Akane's base outfit: a black collared shirt, red plaid bow, white jacket, red armband with the words "SELENE GIRLS SCHOOL", a black pleated skirt with white fringe, black thighhighs with red lines, brown loafers, and a brown vest.
This tag implicates suo_sango (learn more).