Mecha-Ude: Mechanical Arms

Season 1, Episode 5: Are You Hiding Something?

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Aki is finally able to track down the worm-type Mecha-Ude user who killed her family. But the person she finds is her younger twin sister Fubuki, who should've also died in the incident that killed Aki's family. Aki is told the truth about what happened in that incident during her unexpected reunion with Fubuki and she's informed of one other cruel truth.