A disguised major foe comes out and challenges a character, perhaps the protagonist. During the fight, when the foe is satisfied with what he sees from the character, he unmasks, and reveals himself to be...the character's grandfather
! Or some other older relative. Typically they will be the one who gave that character their earliest training.
The characters I am thinking of are Grandpa Gohan from Dragon Ball, and Vice-Admiral Garp from One Piece. Both first appeared masked, and discarded those masks for a surprise reveal after fights.