Or known elsewhere as Masked Rider Black or Kamen Rider Black. This was your afternoon 4PM show while you were young. You caught every episode, although you can't remember what it was about. Here's a recap:

He's a normal guy, but transforms into the Masked Rider Black. He's a superhero. Watch him transform:

He basically fights monsters that are human-sized. Consider him a mini Ultraman, who rides a bike.

His backstory was pretty deep. Kidnapped by the cult Gorgom on their 19th birthday, stepbrothers Kotaro Minami and Nobuhiko Akizuki were experimented on and were fused with cyborg surgery so they could become the next Gorgom Creation King.

The two are destined to fight against each other in a final battle. Kotaro was determined to rescue his stepbrother from Gorgom while protecting Japan from monsters.

Check out an episode below: