Kaze no Kishihime(2009)

Kaze no Kishihime

Kalina's dream was to become an elite knight, just like the one who saved her when she was little. But women aren't allowed to join the elite Mage Guards. So she changed her name to Kalin, dressed up like a man, and went to the capital, where she got into an argument with a nobleman named Centurion that escalated into a duel! Kalina was accepted into the Mage Guards as an apprentice. But that annoying Centurion was assigned to be her superior and her roommate, too! Can she hide her real gender, and become the knight that she dreamed of?

(Source: MF Bunko, translated)