Vampire Panic

Unlock Abbey
Rescue the few villagers in the wilderness after visiting the Sanctuary initially. Use Rusty to inspect the back wall of the Sanctuary's Main Hall and you will discover a new area. Locate Abbey in one of the unlocked jail cells in the dungeon area. Take the little girl to the cell and Abbey back to the Sanctuary. You may now play as Abbey.
(Thanks to osgl_second_nature)

Unlock Hard Difficulty
Clear the game on Normal difficulty or unlock Weapon Inheritance.
(Thanks to osgl_second_nature)

Unlock Second ISLA Ending
Clear Story Mode once for ISLA, to unlock Vampire Side. Save over the Vampire Side game once the Vampire Side game is cleared, and start a new ISLA Side game on Normal difficulty. A new boss will be available in the last scene. Defeat this boss.
(Thanks to osgl_second_nature)

Unlock Vampire Mode
Clear the game on any difficulty and with any outcome.
(Thanks to osgl_second_nature)

Unlock Weapon Inheritance
The inherited weapons for Rusty, Mary, and Abbey can be carried over to subsequent new games as long as the following is done:
Clear the game on any difficulty to unlock Vampire Side.
Clear Vampire Side.
Clear ISLA Side on Normal difficulty.
(Thanks to osgl_second_nature)