HachimaJin serves as the main antagonist of MegaTagmension Blanc + Neptune VS Zombies, and the cause of the zombies plaguing the school. It is said his being was created from people's Dark History. Defeating him can get you the HachimaJin Parts.
The first time that HachimaJin is mentioned is during the second half of the game. Uzume tells the rest of the cast that his seal had been broken and she has been tracking him for some time. She then tells them she plans to destroy him once and for all. Blanc agrees to this, on the condition that Uzume helps them with their movie.
Upon hearing HachimaJin's origin story Blanc work's it into the movie plot. Several evil organisations had created a zombie virus using HachimaJin's DNA, which had spread across the island, and turned everyone except the main heroes into zombies as well
HachimaJin will attack either with his hands or the large cannons on his back. With his hands, he will swipe at you if you are in close range. Using the cannons, he will either fire them into the air and have bombs come down from the sky, or he will fire lasers across the field. You can attack different parts of him, however, to defeat him you need to destroy his body. His hands and cannons can be destroyed, but will both regenerate fairly quickly. Once you have done enough damage to him, he will be stunned for a few seconds, which will give you a free run to attack him. EXE Drives work well at these times. HachimaJin is weak to Slice attacks so having Slash Patches is helpful