Kikan

When she is 14 years old, the hero's sister vanishes without a trace on the way home from school. Several years later she returns, apparently unharmed and with no explanation of her time away. She has, however, gained two interesting new traits since her disappearance -- the first is a strange tattoo in a sensitive area spelling "αλαωε" (her name, presumably, in Greek letters). The second is a ingrained trigger that forces a compulsion in her -- whenever she hears a ringing sound, she turns into a nymphomaniac who cannot be satisfied until she passes out!