Kazuo Koiki passed in April of this year and was well-loved as one of history's greatest manga authors. He lived a long life, being born in 1936. He's mostly known for writing Lone Wolf And Cub and Lady Snowblood, which appealed to the Seinen demographic mostly made up of older male manga consumers. Both of these manga series were well-received after their importation to the west by Dark Horse comics.
He was also the recipient of a Hall of Fame Eisner Award in 2004. He cut his teeth in the manga community by writing for the creator of Geigo 13, a popular Seinen written by Takao Saito. While those two may be his biggest series, there are plenty of underrated series by this late and great author. This list will outline the others you should check out as a fan of his work.
10 Lone Wolf And Cub
Lone Wolf and Cub has been made into 6 movies, 4 plays, and a TV show. It tells the story of the executioner of the great Shogun who's had his family name disgraced after false accusations by a rival clan. Since he can't operate in the open anymore to exact his revenge, he must become an assassin and with his 3-year old son, exact revenge on the Yagyu clan. It's one of the most well-respected manga ever written thanks to its historical accuracy, its careful treatment of the Bushido code of conduct, and it's broad scope.
The lone wolf's cub also has a character arc all his own since he grows up throughout the length of the manga. The series influenced the creation of Frank Miller's Sin City and Ronin, heralding western comics' obsession with samurai and ninja that these series would crank up into high gear for the duration of the 80s and 90s.
9 Lady Snowblood
Lady Snowblood is a manga about a woman who seeks to wreak vengeance against those who disgraced her mother and murdered her father, therefore dishonoring her family line. She often reclaims her sexuality by using her feminine wiles to seduce or distract her foes.
The Japanese title makes references to the Asuras, a race of divine beings right below the gods in Hindu and Vedic mythology whose only drive is revenge and jealousy towards the lavish lifestyles of the gods, fighting an eternal war against them, which they always lose. Lady Snowblood and the film made about in 1973 have been explicitly cited as major influences on Quentin Tarantino's Kill Bill films, which makes sense when comparing the plots.
8 Crying Freeman
Crying Freeman is a psychologically troubling series about a man who becomes an assassin for the Chinese mafia after having taken from his homeland in Japan and being brainwashed into becoming a killing machine.
To show his regret, he sheds a tear after every life he takes against his will. He starts the series as a mild-mannered potter, and since he has no way to escape the fate that the Chinese mafia has created for him, we see a serious struggle within him about how he's been placed into this dark and involuntary servitude.
7 Samurai Executioner
Samurai Executioner was created by the same team that made Lone Wolf And Cub but is much shorter at only 10 volumes. It's about a ronin named Yamada Asaemon who's in charge of making sure that new swords are up to snuff for the Shogun.
The story is again known for its historical authenticity, since these sword-testers were actually real people, usually being charged with performing the executions of criminals and political rivals to the Shogun. The stories are actually mostly about the deeds that led his victims to execution.
6 Path Of The Assassin
Path Of The Assassin is a Gekiga, meaning "dramatic picture" aimed at adults rather than young teens. It is the first of the crew behind Lone Wolf And Cub's manga to focus on actual historical figures.
It's about Hattori Hanzo, a man who's tasked with protecting the child who's destined to become the next Shogun. If that name sounds familiar, it should because it's the name of the swordsmith who makes a sword for the bride in Kill Bill.
5 Hanappe Bazooka
Hanappe Bazooka is a rather strange one for Koike since it isn't historical, there are no ninjas or Shogun or samurai to be found.
It's about a kid who has a portal to a realm of demons open up in his house, which ends up plaguing him but also giving him the power of the Hanappe Bazooka, which is both a death ray and will drive women insane with lust. Definitely a departure from the normal stuff.
4 Hulk: The Manga
It's so nice to see the cross-cultural effects of working on fun properties at play here, specifically between the United States and Japan. Hulk: The Manga is a series done by Koike that unfortunately never got translated.
While it's difficult to check out unless you speak Japanese, the art is a really interesting thing to see, and apparently, if it tickles your fancy, there are also a few Spider-Man stories that got this treatment.
3 Mad Bull 34
Mad Bull 34 is a story about the toughest cop in NYPD's 34th precinct Joe Estes, who received the name of Mad Bull, and his partners. It's an action manga, but given the plot that's been spelled out, you can probably guess that it ends up being pretty comical at times, even if that comedy is just a little dark.
Mad Bull isn't afraid to kill mild-mannered criminals who don't really pose a threat with a shotgun when he feels like it, but it's always revealed that Mad Bull had good intentions all along at the end.
2 Wounded Man
Wounded Man is a tale fairly similar to the first portion of Frank Miller's Sin City.
A man's wife is kidnapped and murdered after being forced to act in adult films, and Wounded Man details the revenge of her ex-lover, going after anyone who had a hand in making that happen.
1 Kawaite Sourou
There's a new Shogun in town, after a long period of regime change and an unstable political future.
When a handsome and mysterious man enters the town and becomes the newly-appointed poison taster, it suddenly becomes very clear that the lives of the Shogun and his poison-taster are in danger, and that maybe the political future of Japan is in danger.