According to Wikipedia, the 13th assassin is not human and it is "hinted" that he may be a forest spirit. I read a
thread in IMDB, which was a bit more informative. Makes me want to catch the full 141 min version to see what other blood and gore I missed. Coming to your question on why a forest spirit should assist the 12, it may be just the super-human element they needed to take on an army of 200? As a spirit, he knew he
had to do it? In the 141 min version, it is said that they offer him money and ask him to leave once they get out of the forest, but he refuses and stays with them. If he is just a human with great strength to survive sword cuts, then he stayed just for fun - like Naritsugu having 'fun' in all the bloody mess around him. Is there any other better explanation you can think of?