Mastermune

 is Serge's ultimate weapon in. It has a 100% critical hit rate. It can be found after defeating Dario on the Forbidden Island. Mastermune deals extra damage to Demon, Dragon, Floating, Human, Marine, Matter, Soft, and Zombie enemy-types. Mastermune may be an allusion to the Triple Tech of the same name from.

Transformed from the holy sword, Masamune, its inhabitants (Masa & Mune) awaken and dedicate themselves to be helping Serge. Doreen melds with them to form the weapon. When it is equipped and Serge uses his FlyingArrow tech, Masa & Mune can be seen behind him powering it up.