666 Satan, Dragon Ball Recommendations
This Anime reminds me a lot of Dragonball The boys looks nearly the same, turn both into a monster, are strong,their first friend was a girl and then their adventure starts. Actually Ruby looks really the same as Bulma hair eyes and body. Both of the stories continiues with killing enemys, finding friends and killing the biggest monster.
Although they develop in entirely different directions, the beginning in both series is practicaly the same. Wild, strong boy with mysterious past meets a young girl, decide to go with her on a journey as a bodyguard and eventually becomes her friend. Well, these mangas are similar in many ways and both are fun to read, so if you've just finished reading O-parts hunter, You should definitely try Dragon ball.
Both have a lot of similar characters, Goku and Jio, Bulma and Ruby, Ball and Krilin... Both begin as adventures by kids but they grow up along the manga and find more serious and dramatic situations.
In the early volumes, the graphics are a bit halfway between Naruto and Dragon Ball. Jio is practically like little Goku, Ruby like Bulma.
I would recommend O-Parts Hunter if you like Dragon Ball and Naruto. I don't know its popularity anywhere else. Supposedly, it's in the works for an anime release sometime by the end of this year or at the beginning of next.