"What expressions lie hidden behind his mask? ...And more importantly, who is he?"
The Masked Man is none other than Claus (クラウス) the older twin brother of Lucas. Claus looks very similar to Lucas other than the different colors of his clothing and hair, though he is energetic and far more courageous than Lucas. The Masked Man is Claus' robotic Double.
Lucas wears the shirt of the Masked Man in Super Smash Bros. Ultimate in memory of The Masked Man and now it can be yours!
Pixel X Challenge: Who were the first four characters in Super Smash Bros. 64?