To unlock it, you are required to fullfill two conditions first
1. Beat Sabu, the games regular final boss. This will
unlock the Hasabe pendant.
2. Destroy all of the 25 Sabu statues hidden in the game.
This will
unlock the Manami pendant.
Equip both at the same time and enter Sabukos boss room in the Sanwakai Tower to trigger the secret boss. The fight itself is pretty easy, nevertheless, bring some heal items* with you, just in case. As the fight with Sabu, it will have a additional fourth phase. Ignore Hasabe and focus on beating Mamis angel.
* I recommend the video game cartridges that can be bought in Crosstown. 100% healing for 100$ each.