Dynamo

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Dynamo (not to be confused with Dynamo Man in the classic series) first appeared in Mega Man X5; he was sent by Sigma to knock Eurasia out of orbit. He appears twice--in both cases to interfere with the mission (whether it's getting the Enigma ready or preparing the Shuttle).

Dynamo appears again as a secret boss in Mega Man X6, once Gate's Lab is unlocked.