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!
(Source: Yen Press)

Shou, the older brother, and Saya, the litlle sister, only have each other. The two of them lived happily, enjoying their peaceful lives together. That is, until Saya was bitten.

When bitten by a pure-blooded vampire, the one who was bitten may themself become a vampire, as was the case with Saya. She can't leave the house out of fear that she will feast, and her brother must lock her away to protect her from vampire hunters, such as himself. The only solace she has is being taken care of by her beloved older brother, whom she knows has eyes only for her, and vice-versa.

And so, Shou has made it his goal to kill the pure-blooded vampire who bit his sister, believing that in doing so he may restore his sister to what she once was, and becomes an expert for that. But since Saya was bitten, she has become increasingly obsessed and forceful. She can't stand being mere siblings anymore, and Shou is having a hard time holding back.

Will Shou give in? Or will he kill the pure-blooded vampire first? And even if he does, will that be enough to stop the romance forming between the two of them?

Following the untimely passing of his childhood friend and his little sister, Io Haruomi is constantly haunted by their deaths, forever feeling powerless and unable to escape the guilt he feels for letting them die. Despite this, he continues to press forward, attempting to enjoy an ordinary life. He goes to high school, hangs out with his friends, and generally passes himself off as a normal 2nd-year high school student.

One day, however, Haruomi encounters an "Eda", a girl with glowing red eyes and demonic powers. Amidst her escape from the "Seven Great Gospel Organizations", she reveals that he too is an Eda, and the powerlessness he feels is completely unfounded. Before he knows it, Io Haruomi is thrown into the middle of a battle between man and devil.

And as his eyes too begin to glow red, the side he is on is no longer so clear.