Utsumi's Twins

Utsumi Kali gave birth to a pair of twins in August 2011, a month after being confined to the Third Area in Hotarubi Mine. The twins are involved in the personality swaps and are a cause of much of the story's tension. While not given official names, the male twin is called "α" (alpha) by Satoru when he debates the existence of a third personality in the swaps. Similarly, the community refers to the female twin as "β" (beta) or "ω" (omega).

Personality Swaps
The twins are part of the two personality swap cycles, namely:


 * Kokoro -> Satoru -> "α"
 * Hotori -> Inubushi -> "ω"

Since their minds are still underdeveloped, they cause much trouble for the cast without realizing it. "α" causes Kokoro's body to tear up the newspaper and eat the emergency rations. At the same time, he causes Satoru's body to go on a rampage on multiple occasions, drinks both the DMT and MAO inhibitor, and kills Enomoto Naoya on the 4th day.

"ω" doesn't have much opportunity to cause trouble since Hotori's body is frozen, leaving only Inubushi's body to control. She plays outside of Satoru's bathroom on the 2nd day and splashes around in Enomoto's blood on the 4th day.

Together, they cut Satoru's hand during the blackout. and can cause Satoru's body to drown on the second day.

Location
Since the bodies of the twins are "asleep" within Utsumi's womb, Kokoro and Satoru are unaware of their existence despite transferring into their bodies. However, the player does see them inside the womb. There are multiple references to this:

On the first day, Satoru finds himself a "primordial sea":
 * "Extending a sensory organ, which was neither an arm nor a feeler, I tried to search for my body.
 * The thing I called my body was nowhere to be found; only a soft, flabby aggregation of amino acids surrounded me.
 * I began to melt into the boiling water, as the water also melted into me."

On the fourth day he comments on it again:


 * "I've had this dream before....
 * The primordial sea....
 * I can no longer tell where my own existence begins or ends.
 * I felt as if this whole vast sea were myself, and that there were no singular entity in existence here.
 * My surroundings were completely saturated by a dazzling red light, a rhythmic sound, and a sweet scent that felt as if it would dissolve...."

TIPS 099 spells this out clearly:
 * ""Primordial sea" is a metaphor for the amniotic fluid in the mother's body which surrounds and cushions the fetus, softening the shocks from outside."

Trivia

 * The twins mirror Yukidoh Satoru and Yukidoh Sayaka, who are also fraternal twins. Considering that Sayaka's personality was overwritten by Inubushi's, that means there are two pairs of twins cycling around in the personality swaps. The only outliers are Kokoro and Hotori. However, Kokoro was also born on the dame day as Satoru and Sayaka.