Skull Man

With a dark gothic pallet for the animation and similar styles, both of these series involve a protagonist who is an antihero trying to do the right thing, but fate doesn't want to let them be just a hero and instead forces upon them a power that forces them into the role of antihero instead. 

You could say that "The Skull Man" is Japans dark heroes tale in comparison to America's "The Batman". If you like dark hero series then the one you haven't watched may be for you. 

There's a dark secret behind the main protagonists despite all heroic intentions, but the secrets surrounding them are like an abyss, but certain individuals don't appreciate the heroes because of the destruction which follows. 

Masked men, mysteries, action, supernatural or sci-fi genres. Both are highly enjoyable series for the fans of mentioned themes. 

Most series has a similar dark feeling and tone regarding the mysterious events going on around the setting. Both series features a cast of characters that comes into contact with the mysterious title character in mysterious ways that often ends with violence. As such, both series has violence, drama, and a very serious tone throughout each episode. It is a watch for fans into the psychological and thriller like sequences. 

An ensemble cast of characters, including a main reporter lead and his female sidekick, has a role to play in a mystery spanning years, which has suddenly mutated the people of a city into unknown abominations. Both 13 episodes, these two shows are practically the same. 

Skull Man and Boogiepop And Other are about mysterious events occurring throughout a city. There are a wide variety of characters, with many of them having extraordinary talents. Both series have the same tone and can be very confusing at times, more so in Boogiepop.  

Although they are different genres (with Rapeman being a hentai), both present the superhero genre in a unique way. The main characters of both anime are masked vigilante who patrol a dark and corrupt city, also dealing their own brand of justice. Also, both main characters have alter egos that have respectable day jobs. 

- A dark hero who fought to protect humans from the attacks of monsters but people judge him as a villain to be destroyed. Making garo & skullman almost like anti-hero. - In both films, the city where the story takes place is actually ruled by antagonist side. 

Both are about people (believed to have died) who come back to destroy those who wronged them in the past 

They are both full of mystery and keeps you thinking and making guesses.  

Both have dark stories with tragic heroes fighting monsters. Both feature henshin heroes (The Skull Man originated Kamen Rider) and have a tokusatsu feel to them. 

Hey you, yes you, do you like action? Do you like Japanese style Henshin heroes such as Super Sentai or Kamen Rider? That my friends is what these two shows have in common. What they don't share however, is tone. Sadamitsu the Destroyer is a lighthearted, almost parody of Tokusatsu style shows, while Skull Man is a serious, dark take on similar concepts. Additionally, Skull Man came out six years later then Sadamitsu and was animated by studio Bones and not studio Deen, meaning it has far, far better animation. But, if you enjoyed one, or tokusatsu shows in general (which is probably why you ended up here in the first place) you'll most likely enjoy the other so long as you go in with the right expectations. read more 

The original mangas for both The Skull Man and Cyborg 009 were created by Shotaro Ishinomori. Joe Shimamura/Cyborg 009 even makes a camero appearence in the Skull Man manga to debate evil with him (since they are the hero and anti-hero interpratations of the same idea at heart). The Skull man is considered a pseudo prequel of Cyborg 009. 

Dark Retelling's of Shotaro Ishinomori classics, each one retells the story of a robotics based henshin hero with a more darker tone than its original. Although the dark I'm referring to for The Skull Man is more of a horror story dark while the dark for Kikaider the Animation is more of a emotional and a tiny bit film noir style dark 

Both shows have a similar feel and theme - a dark story of humans turning into monsters. They both have nice animation and both shows get better as they get further in. 

MC comes to isolated city for solving complicated mystery. Great realistic characters and very interesting story. Both oriented on mature audience. But one is more calm and slow, other is more dark and dynamic.