HP, or Defense points, is the points needed for the characters to sustain fighting against the Oni (Steve) during boss battles. The HP decreases when the Oni strikes back. If the HP of a character falls to zero, the character will be shown to fall to the ground "dead", unable to fight anymore. If all the characters have zero HP, then it will be game over.
The character could be revived by offering him onigiri, which increases his HP by 500. Alternatively, the user could select to use Italy's skills instead. Carbonara restores the HP of one character and pescatore restores the HP of the whole party.