We've all seen/read countless stories where the hero has to fight the villain, and sometimes that villain is his father, grandfather, older brother, ancestor, etc. We can name many, many examples of this kind of story.

Are there any stories (preferably Japanese manga) where the villain is a younger relative of the hero? What if the hero had to fight/kill his own son/daughter, younger sibling, etc.? I don't think I've actually seen anything like that before.

If I've made a thread in the "I'm Looking For..." forum, I am only interested in Japanese manga. I am not interested in non-Japanese media at this time.

Cavalier of The Abyss. - Start reading with it's prequel Immortal Regis and you'll see how it is. There are many twist and turn. And the hero position is shared, because of two books. But, they are relatives.

Try Witch Hunter and D. Gray Man.
The scenario is a bit different, but may also give Tenjou Tenge a try.

Images are disabled Images are disabled Tv Tokyo - Anime & Manga ~ MinatoAce

I don't know if this is going to count as a spoiler or not.
It's Enen no Shouboutai, from the same person who did Soul Eater.