Koizumi is petite with red eyes and blonde hair. Her hair is normally long and wavy, but is fixed to a ponytail whenever she eats ramen. She is generally quiet and reserved. Although Koizumi has a penchant for ramen, she prefers to eat by herself. Koizumi often carries with her a dull, emotionless demeanor, but upon eating ramen, a cheerful and satisfied expression will often light up on her face. Her manner of speech is polite and straight to the point.