It's a manga series.
The first series named Kotaro is rather old, but fuckawesome.
The second was Shin Kotaro, the first one was karate but mostly just bloody freestyle fights, the second is about judo.
The third was Kotaro F, it's about ninjas, and the protagonist's mom.
The one posted here seems to be the third one.
No hentai, just ecchi stuff.
It's rather unpopular.