This is a handmade doll that Kokoro made. Due to her desire to become a mother, the doll resembles a baby.
Following the discovery of the Baby Book in Episode 07, Kokoro asked for a sewing kit as her annual present from Papa in Episode 09, which she used to make a baby doll. The doll is first seen in Episode 11, while she is humming to herself and waltzing in the greenhouse with it and asks the doll if the flowers smell nice before she is found by Mitsuru.
It is revealed that that the reason that Kokoro made the doll is that she longs to bear a child and she is seen playing with the doll in one of the abandoned rooms in Episode 17. Sometime after she and Mitsuru begin an intimate relationship and sleep in the room, she keeps the doll in the room with them. It is seen next to Mitsuru while he sleeps and is later found by Miku, who brings it with her to the Bird Nest. After Kokoro’s memories are altered, she has no recollection of why she made the doll. However, after becoming a mother, she gives the doll to her children.
In the ten-year time skip, the doll reappears while Kokoro and Mitsuru attend their children’s school entrance ceremony.
- nocandnc's baby doll theory on tumblr
- The doll is shown to be a female, which is seen as a foreshadow, considering that Kokoro and Mitsuru’s first child is a girl.
- The hair colour of the doll closely resembles Mitsuru’s hair colour, strongly hinting that, all along, Kokoro wanted to bear a child fathered by him, as it is revealed in Episode 16 that she is in love with him.