The Siren is a boss enemy in the late game. It is fought in the area after the elevator ride with Sotiris. To move on, the player must kill the various talking brains located in areas guarded by The Siren, eventually having to kill it in the last area. This serves as a boss fight and puzzle obstacle. It is a very big enemy, often blocking the player's path. It's fast, but can be easily outrun by the running player.
All of the notes near the talking brains that summon the siren are references to ways to kill it in the given parts: eg. the first, being electrocution, and the last getting crushed.
The Siren looks like a humanoid with large hands. It doesn't have a nose, instead having an exposed nasal aperture. It also doesn't have eyes, possessing two horizontal, feminine-looking smiley mouths. It also doesn't have legs, having to crawl to chase the player.
Area 1, The Nasty Old Man
After the player kills The Talking Brain, they see a vision of Virgil summoning The Siren from the depths. After that, it will start pursuing the player. The player must run to the bridge and activate the breaker box, shocking the creature and making it retreat.
Area 2, The Nice Old Dog
After killing the second talking brain, the player gets into what seems their dog's view, guiding the player through the first area of the game. The area where the homeless man resides becomes the flood gate's opening, And entering that area will cause the vision to go away, and go back to the player's point of view. After that, behind the flood gate there will be a loud screech heard, and at this point the player has to activate the flood gate to make the Siren go away. If the gate isn't flooded, the siren will destroy the door and kill the player.
Area 3, The Great Old Beast in the Great Old Wood
After killing The Talking Brain, The Player will be ambushed by The Siren and the path back out will be blocked. The player must defeat The Siren by shutting a large door right on it, and attacking The Siren in order to proceed. Then, Childhood Memories will pop out of it's face, dying in only one shot.
Considering the name of the baby humanoid (Childhood Memories) this monster is likely dedicated to getting over PTSD caused during the player's childhood, being the last "thing" to get over during the game's play time (the other ones being self-image and such)