Ryusei no Kizuna (2008)

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"When we grow up… We'll kill him together". Koichi, Taisuke, and Shizuna are three siblings whose parents had been brutally murdered when they were just in elementary school, and they vowed to avenge their death. Fourteen years later, their vengeful plan unravels countless new facts. A shocking revelation leads to an overwhelmingly emotional ending. What will become of the siblings who have lived their lives solely in reliance of their bond alone? --TBS