Asterisk (manga)

Asterisk (アスターリスク) is a manga by Michiyo Akaishi.

Story
Ayame Yagami (八神 あやめ), teenager, follows her sister, a seer who seems to be fleeing from something, from town to town; they've got no parents, as both had been killed several years before, and they've got very few friends, as they rarely stay in the same city for long. Her only childhood souvenir is a key holder given to her by Shū Mizuhara (水原周), which he found at her old school.

Suddenly her life changes: Shū and his friend Hikaru Tsukumo (九十九光) reveal to her that they're part of a demon hunter association called Asterisk. Ayame discovers that her sister is a demon, before realizing of being a demon herself, with golden horns; her sister is killed by Hikaru and his father, and in his vengeance she kills the father and makes Hikaru lose one of his eyes, then flees from the other Asterisks. Shū, in love with her, will follow her, betraying Asterisk.