Omamori Himari

Synopsis

From Yen Press:
Severely allergic to felines, Yuuto Amakawa spends his mornings nursing a drippy nose courtesy of his friend Rinko’s cat. But on his sixteenth birthday, it’s not just Rinko’s pet that’s riling up his allergies (to say nothing of his hormones)! Appearing before him is Himari, a buxom, sword-wielding cat spirit in human guise, who has sworn to protect Yuuto, now the target of vengeful spirits! It’s clear that Yuuto’s allergies are the least of his problems he’s gonna need Himari’s help if he wants to live to see his seventeenth birthday.

Synopsis & details from MangaUpdates