Omamori Himari

Yuuto is your average, high school boy just turned 16. His childhood friend, Rin has taken care of him ever since his parents died when he was younger. All of a sudden, a girl named Himari walks into his life. Himari is a cat-girl samurai who is sworn to protect Yuuto. There's only one small problem: Yuuto is allergic to cats!

(Source: Manga-Heaven)