Yomika is a Ghost
This theory contains spoilers for the game's ending.
The chasers in the game all look like different colored versions of Yomika but have one part of their head detatched. Their walking animation looks more zombie like and they walk with their mouths open.
A lot of in-game events have something to do with the chasers and so does the good enging of the game. It is very likely that Yomika and the chasers are closely related.
The good ending
In the good ending of the game, Yomika will find big structure that resembles a lock. She will start crying and disappear into a white flower.
However, on close inspection you can see a shape on top of the structure that resembles a chaser, with the same part of the head detatched. Because the chasers can only be found inside the dream world, the one on top of the structure may be Yomika's actual body. Yomika is a ghost who does not know or does not want to accept that she is dead.
When she realized that she is dead, she cries and is able to move on.
There is also a wake up event when you go to this place but in the dream world. Yomika will find a dead chaser in the same spot as the structure is in the real world.
The bad ending
In the bad ending, Yomika needs to go into the opposite direction of the structure. She will wander around the area aimlessly until the screen completely fades to black.
In this ending, Yomika's ghost does not realize or accept her death, symbolized by going the opposite way of the structure and facing away from it.
Her spirit will wander around forever, unable to move on.
Yomika's name
Yomika's name might come from Yomi, the land of the dead in Japanese mythology.