It is recruitable or fought at the Great Prairie. After defeating the Haniwa Horde, it disappears.
Haniwa has very low HP, and it doesn't grow, either. This is compensated with the Haniwa Suit, with 82 Defense and Magic Defense.
This thing possesses the Seven Branched Sword, a 74 Power special weapon, it is in fact very powerful, as Haniwa has excessive Strength(26).
Like Zo, it can only use Earth, Sun and Moon Magic.
Haniwa isn't really a party member because of the events it is submitted, as it dies in all of them.