Seigi wo Aisuru Mono Gekkou Kamen

As the theme song denotes, no one knows who Gekko Kamen is. He is a lone figure dressed entirely in white, who mysteriously appears when danger is afoot.

While he appears to be a regular human, he is exceptionally gifted in many areas including unarmed combat and gymnastics. He is an expert motorcycle rider and a superb marksman.

(Source: AniDB)