Shin Hisui no Shizuku: Hiiro no Kakera 2 Portable

Suzu's older-sister figure, Mao, has vanished. Summoned back to her rural hometown to take up Mao's role as Tamayori-hime — the sacred miko destined to sacrifice herself to restrain the sea lord — Suzu inherits both the duty and the mystery. She barely begins to understand the role before the sea lord attacks, sending his demon forces against the village. Leading those forces is Toyotama-hime — the face underneath which is Mao's.