With Japan's long history of comic books (aka Manga), they featured their own versions of dark heroes, hot blooded heroes and cynical heroes. The question, is there a hero that could be considered their own Superman?
Most tend to see DBZ's Goku as such, due the fact they share a similar background (born on a distant planet, sent to earth as an infant, home planet destroyed, raised on earth, grew to become protector of Earth), however Goku doesn't have a secret identity (ironically his son Gohan did when he became the Great Saiyanman), has a more child like personality and apart from his fighting smarts, isn't exactly the brightest character in the world but he has a likable upbeat personality. Still he tended to kill his opponents (not a prefarable action but most of the time he had no choice) Possibly more comparable to Golden Age Superman.
Next on the list is live action hero, Ultraman, another character who recieved the title of Japanese Superman from fans. He came from another galaxy far away from Earth, has a secret identity as a form of a human who was close to death, therefore becoming some sort of lifeforce for his host however some Ultra heroes merely created their own human bodies to take cover as and when monsters go on rampage the human form transforms to Ultraman due to a certain object. However like Goku, every Ultraman (apart from Cosmos) tend to kill the monsters. Again more comparable to Golden Age Superman (and Golden Age Captain Marvel, being a transforming hero and all)
However in my opinion, Japan's true answer to Superman is Osamu Tezuka's Astro Boy.
While not as powerful (even he was powerful enough to defeat an evil sorceror in the critically acclaimed GBA game, Astro Boy: Omega Factor), plus the fact Astro isn't an alien, but an A.I and he has no secret identity, he can be comparable to Superman for several reasons. While Superman is seen as the grandaddy of comic book superheroes who set the standards that others would follow despite not being the first comic book hero, Astro is seen as the one who set the standards for other manga titles to follow despite not being Japan's first superhero. When Sups had the Flecther shorts that set the standards for animated superhero films, Astro's 60's anime was the first anime to have a more continuous formula (with 2 parters and such). Therefore in terms of importance in their own countries, they share a similar influence.
Like Superman, Astro tries to avoid killing his enemies, he usually stops robots from killing humans or destorying eachother. He is optimistic and kind hearted. Like Sups, Astro can be seen as more universal comic character due to Tezuka's anti-war sentiments. Although in the short manga series "Ambassador Atom" where Astro made his debut, he showed his more violent side near the end when he found the Earthlings decision to attack an innocent alien race, illogical (he didn't kill anybody but still), since this was his first appearance, this version could be comparable to the more violent Golden Age Superman. In that short manga, Astro would sport a more traditional superhero attire (cape included), of course none of that made it, when he got his own manga title although he still wore his red boots.
Unfortunatly like Superman, Astro has been a target of criticism by cynical fans who saw him as too idealistic and a tool of establishment, boring and bland therefore prefering more darker, loudmouth or cynical manga heroes, which was trully evidented in the 70's when more cynical, deliquent like manga heroes were big sellers and Tezuka was forced by his publishers to make Astroboy less moralistic which ironically caused sales to drop, therefore gave Tezuka the chance to make Astro the way he was again.
Of course like Superman, Astro had a pretty big impact on popular culture in Japan and abroad. While Ultraman and Goku have made strong impacts in their own rights, Astro Boy's celebrated as something of a national icon, going as far as getting registered by the city of Niiza, Saitama as a resident with on April 7, 2003 to coincide with his birthdate in the manga as as well as the 2003 remake.
Astro remains as one of the few anime character's who's story was retold and revamped. He had 3 anime shows in the 60's, 80's and 2003 and one of the more iconic stories "The Greatest Robot In The World" was remade by renowned manga writer/artist and multiple Eisner Award nominee Naoki Urasawa in his wonderfully drawn and written Pluto. There are plans for a live-action/CG film in the works from Illumination Entertainment and Universal Pictures, the production team responsible for the 2010 summer blockbuster Despicable Me.
Apart from the 2009 American CGI adaptation, Astro was also adapted into American comic books several times, the most notorious being the 1987-1989 NOW Comics version
A nice little trivia for you all, Tezuka was also a big Superman fan and was made honorary chairman of the Superman Fanclub in Japan during the release of the Christopher Reeve film in Japan.
So this was my opinion of why I think Tezuka's iconic character deserves the mantle as Japan's answer to the Man of Steel, then again it's just my opinion.
Now as a way to end this article, here's a funny little picture
The last picture was done by DragonRider1227 http://dragonrider1227.deviantart.com/