The default costume of Nijisanji's Nishizono Chigusa.
It is the group's Selene Girls school uniform: a black collared shirt, blue plaid necktie, grey hoodie with an ebi print, white jacket, armband with the words "SELENE GIRLS SCHOOL" (colored blue for Chigusa), a black pleated skirt with white fringe, white sneakers, and a dolphin hair ornament.
This tag implicates nishizono_chigusa (learn more).