The Asterisk War, Vol. 2 (light novel): Awakening of Silver Beauty — Yuu Miyazaki

Having formed their tag-team partnership, Ayato and Julis begin polishing their skills and their teamwork in preparation for the Festa. But soon Ayato meets another girl--Kirin Toudou. She has long, silver hair and a petite charm--and despite her thirteen years of age, she's one of the strongest fighters of Seidoukan Academy. She seems to like Ayato, but that's not going to stop her from challenging him to a duel...