Bettou Izumi Shizuka Scarlett

A half-American girl who belongs to a spy family. Shizuka is Kurou’s younger sister and Akito’s love interest. She has a somewhat devilish attitude at times but is generally a cheerful person. She can fly most aircraft in the world, is fluent in 6 languages, and has the habit of threatening Akito with her 8mm.

Edited from a combination of Haeleth's post and Choux's review.