The Whispers spin in a tight clockwise group around a single point while slowly chasing Isaac. They deal contact damage and float over obstacles and through walls. They can perform the following attacks:
Teleports to one side of the room and spins faster while expanding the circle's radius outward until it covers a room's worth of width. While spinning, the trio then slide horizontally from one side of the room to the other 3 times, ending on the opposite side from where they started. After coming to a stop they will slowly circle back closer together.
Teleports to a corner of the room with their cheeks puffed up, spinning a slightly larger radius while moving very slowly toward Isaac's direction. After a delay, the ghosts each fire a quick, loose line of 3 spectral projectiles along the closest cardinal direction to Isaac. They will repeating this attack 2-3 times.
The Whispers share a single health bar, attacking and damaging any of the ghosts subtracts from it. When it's depleted, all the ghosts die together.
Notes
As The Whispers are classed as "ghosts", using Vade Retro while they are at 50% health or less will instantly kill them.
The boss rooms of The Whispers will always be layouts filled with various obstacles. This is likely to compensate for the weaknesses in their attacks as they would be simpler to avoid in an open room.
The Whispers' spin attack and projectile attack follow set locations that is determined by the room layout. In some rooms, their spin attack can make a loop around the room, rather than moving back and forth along a path.
A single attack that hits multiple ghosts at once can deal a great amount of damage, meaning a single well-placed Bomb explosion that hits at least 2 of the ghosts can deal over 200 damage, which is well over The Whispers' maximum health and will typically kill them instantly.
The Whispers are a direct reference to Boo Rings from the Super Mario series.
The laugh the Whispers make when starting their spin attack is a sped-up version of Satan's laugh. This is in reference to Boos from Super Mario 64 having their laugh be a sped-up version of the one from the final boss, Bowser.
Whispers were developed by Fiend Folio developer Blorengerhymes, and were originally created to be in their own stand-alone mod before it was merged with Fiend Folio: Reheated.
According to Blorengerhymes, the three ghosts have no names, as they were not given any.
The Whispers secretly have a fourth ghost that's hidden in the center of the "trio", however it is normally invisible and intangible — it can be made briefly visible by flashing it with the Toy Camera, where it appears to have a small, blushing face.
This could be reference to the easter egg in the game Spelunky, where using the Camera on The Ghost causes it to make a cute pose.
This 4th ghost is merely used as both an HP tracker and pivot point for the other Whispers to orbit around, and as such it does not attack.
Gallery
Ghost #1.
Ghost #2.
Ghost #3.
Secret Ghost #4.
The Whispers' appearances during their spin attack.
Drawing of The Whispers in the Bestiary and on Isaac's last will.
Cellar.
Vade Retro while they are at 50% health or less will instantly kill them.
Bomb explosion that hits at least 2 of the ghosts can deal over 200 damage, which is well over The Whispers' maximum health and will typically kill them instantly.
Satan's laugh. This is in reference to Boos from Super Mario 64 having their laugh be a sped-up version of the one from the final boss, Bowser.
Toy Camera, where it appears to have a small, blushing face.
Bosses 