Ninja Hero (or Ninja HERO) is a martial-arts genre book for the Hero System, published by Hero Games. There are two distinct editions, one for the Fourth Edition of the rules and one for the Fifth.
The original Ninja Hero was written by Aaron Allston. A large portion of the book consisted of a large array of martial arts styles adapted for HERO System play. The HERO system had recently converted from a basic description of martial arts to a more flexible one in which styles were constructed by selecting individual maneuvers. Ninja Hero was the first HERO System book to outline a large number of specific forms in this system, and thus served as a guide for players wishing to create a specific type of martial artist character. This material was followed by a discussion of the genre.
Eventually, the specific arts were split off into a separate book, The Ultimate Martial Artist, as part of the Hero Games Ultimate series.
The Fifth Edition rules retained this division. The Fifth Edition version of Ninja Hero was written by Mike Surbrook and published in 2003. it consists entirely of genre material, further subdivided by campaign style in addition to power level. A fifth edition version of The Ultimate Martial Artist covers the specifics of buying styles."
Source: Wikipedia, "Ninja_Hero", available under the CC-BY-SA License.