Kefla was the fusion of Caulifla and Kale, two female saiyans from Universe 6 during the tournament of power in Dragon Ball Super.
Her fusion served as the last real hope for Universe 6.
She was strong enough to make Goku unable to beat her at Super Saiyan Blue.
So Goku defeats her in probably the coolest move of the series with his rather new Ultra Instinct power.