Big Brother, Where Is This Nursery Rhyme From?

S1, E21: As the young will do, Hazuki and Hikaru rush unheeding into the arms of the enemy. As they seek to escape from the elephant-headed Vargas, Yayoi and Seiji hurry to their aid. Alone and wounded, Kouhei is at the mercy of the child vampire, as the masked defender of the mountain keeps watch from the forest shadows. As one servant cannot serve two masters, sisters will never get along. Sibling rivalry drives Kouhei to tap into his true powers and as the vampires are forced to retreat, innocence is taken hostage.