Source: Siren Maniacs, page 42-43
Siren 2: Creatures, Part 2
Otoshigo
堕慧児
Affected by the shared consciousness of Mother (Yamirei) in the World of Nothingness, the Shiryo (imitating Mother) gather at the bottom of the sea and fuse together to form this. 19 years ago it took form by copying mother and creating a clone of Ichiko Yagura (who died in the sea), appearing as a monstrous, eerie face. Otoshigo plans to return to Mother, but these plans end in failure each time, including 29 years ago (the cutting of Yamijima's undersea cable) and 19 years ago (able only to send a Simulacrum Ichiko Yagura in imitation of the doves). In the present day (2005) it successfully sneaks into the Reflected World after being swallowed up by the phantom tsunami, but is defeated by Yorito Nagai right after the pylon's collapse.
Simulacrum
模倣体
A copy of the Offshoots (doves) birthed by Otoshigo in imitation of Mother. Unlike Offshoots, Ichiko Yagura is the only Simulacrum to appear in the game. Otoshigo, finding the drowned body of Ichiko Yagura who fell overboard the shipwrecked Bright Win 19 years ago, created a "Simulacrum Ichiko Yagura" in her image. Ichiko follows her memories from when she was human, but is gradually taken over by Otoshigo's consciousness and completely awakens as a Simulacrum. Not having any set goal like the Offshoots do, she simply aims for the summit of the pylon, wanting to become one with Mother and return to the World of Nothingless where Mother is, but bugs remain due to degradation and she begins to develop an inhuman, brutal mindset. Since she was created from Otoshigo, Shiryo with a certain degree of resistance to light, she has higher light resistance than doves.
Shibito
屍人
A human corpse possessed by a Shiryo. Since they have a faint degree of control over their bodies they can, while clumsily, wield weapons, but they no longer have any vestige of personality, and are of low intelligence. They are easily fooled by diversionary tactics (such as throwing items), so as long as you have a weapon it is possible to deal damage to and defeat them. However, a new Shiryo will catch the scent of a dead body and enter and possess it anew, making them fearsome, immortal enemies like the Yamibito. Since the Shiryo themselves use the bodies they take over to adapt to being above ground, they have a degree of resistance to light.
Shiryo
屍霊
At the time of the Flood of Light, many Ancestors fled to the World of Nothingness to survive, but some remained on the earth. The Shiryo are the remnants of Ancestors who went into hiding at the bottom of the sea, where light can't reach, and whilst losing their forms evaded extinction. They follow the scent of human corpses and inhabit them as parasites, turning them into Shibito, which has led Yamijima's islanders to refer to them as "impurities" and hate them since ancient times. Along with the dove (Yuri Kishida)'s attempts to begin the Underworld descent, they gather on the island once more, trying to return to Mother.
About the Yamibito and Shiryo 1
Siren 2 and the first game share the same keywords, but some of them have different meanings.
Red Sea
The red water that covers Reflected World Yamijima is, like Datatsushi in Siren, the spilled blood of Mother (a god-like being). However, its properties are nothing like those of Datatsushi's blood, which causes a human to transform upon entering their body, instead closer to a sign showing that a supernatural power is at work. Otoshigo, an inferior form of Mother, no longer has the power to be drawn into the Reflected World, but while using its power to create huge waves the sea is dyed red. (-7:00 "Oddity")
Siren
The siren of the siren that you hear when a red tsunami occurs is a "cry" that is heard when Mother interferes with her supernatural powers. Despite not being as strong as Mother's powers, Otoshigo can cause a siren in the same manner. There are three sirens in total heard during the game (-7:00, 0:00, 23:00). The final siren is the death scream of Mother when she is defeated by Itsuki and Ikuko, the power of this phantom tsunami sweeping the remaining people to various realities.
The Yamibitos' Immortality
Since Yamibito and Yamirei share a kind of lifeforce, (if there is a Yamirei in the area) they can use their strong powers of self-restoration to heal whilst remaining in their human shell (if the Yamirei is eliminated the Yamibito cannot revive for a while, but as long as there are Yamirei the Yamibito can regenerate endlessly). However (like when Nagai blows up Yamibito Okita using TNT), if their human shell is blown to pieces, even a Yamibito cannot regenerate.
The Shibitos' Immortality
To humans the Shibito, who get back up however many times they are defeated, (like Yamibito) appear immortal. However, since the Shiryos' abilities degrade as they adjust to life on the surface, they are hardly able to self-regenerate the way Yamibito do. Upon taking a certain amount of damage the Shiryo will leave its human body (shell), "regenerating" by a new Shiryo taking its place later on. As a result, Shibito cannot be killed.
About the Yamibito/Shibito 2
Yamirei and Shiryo descend from the same progenitors. Let us investigate the relationships of "Mother = Yamibito = Yamirei" and "Otoshigo = Shibito = Shiryo".
The Mekkoju, a Fatal Weapon
(Something like) a Tree of life in the World of Nothingness, created by the influence of Annaki. The head of the Ohta family, picking up a "branch" of it that washed up from the World of Nothingness as driftwood, gave it to the islanders are the Mekkoju that cleanses impurities. The reason it can destroy a Yamibito or Shibito stabbed with it is because it is stimulating enough to burn up the foe's life force in a second, returning it to Annaki. The legend and inscriptions were added later by humans.
Awakened Mother
Since the distant past, Mother has been dreaming, imitating images (of all kinds of things that wash up in the World of Nothingness), and releasing doves (however, no one knows under what circumstances or what appearance this took). The reason these happenings caused nothing big is because before the construction of the pylon on Mt. Shinari, Mother was still in a deep sleep, not acting under any clear volition. Is this her first and last real challenge!?
Mother and Otoshigo's Goal
Mother's ultimate goal is to return to the earth's surface (using a human shell), and Otoshigo's is to return to Mother. Since Yamirei (Shiryo) have a collective consciousness, they share the same mind as Mother (Otoshigo), but after entering a human shell the former person's will begins to intervene (memories and emotions from life faintly remaining), so in exchange for resistance to light the Yamibito (Shibito) have the "constraint" that they must communicate.
The Connection Between Yamirei and Shiryo
Yamirei and Shiryo were originally of the same species, but as a long time passed they both began to change into different things. The Shiryo have conflicting wishes, yearning to return to Mother but also hatred at having been abandoned, following after Mother persistently, but Yamirei see the Shiryo as nothing more than troublesome beings that follow them everywhere despite being inferior. This means that the Yamirei and Shiryo are always at odds with each other.