The Siren Spirits take the appearance of a humanoid Spirit. Their clothing is a matching white shirt and pants, similar to a straitjacket (but with the arms free), that are stained in blood. Their head is red while their eyes and mouth are glowing yellow.
When the Spirits are stationary, their heads spin around in circles. Because their eyes and mouth are glowing, their spinning heads resemble that of a siren.
When the Protagonist gets too close to one of these Spirits, they will stop spinning their heads and begin chasing the girl. These Spirits are relatively fast, so it's not easy to outrun them. However, these Spirits cannot pass through road barriers, affording some protection if the girl can get to them in time.
If the Protagonist throws a Pebble before she enters their sight radius, the Siren Spirit will focus its attention on it for a little while, allowing her to run past it undetected.
Mythology and Theories Edit
The Siren Spirits uniform reminds you of that of a straitjacket, the uniforms of patients with sever mental disorders in the mental hospitals. So it is possible that these Spirits may be that of deceased mental patients.
Another thing is that their heads act like a siren when their heads spin and their mouths & eyes glow. Sirens are usually used for police cars and ambulances. If by chance that these Spirits are indeed that of mental patients, then their siren-like heads may represent the ambulances used for hospitals.