Outgoing and out-spoken, one can say that she is childish or immature. Kaguya often speaks before considering what she is talking about.

Spoiler

Yamai was originally one spirit, but for unknown reasons or causes that may be related with her coming and going between the dimensions sometime in the past; she was split into two bodies and mind. The original personality of Yamai is no more, as well as what type of personality she used to have is unknown to the split Yamais. The Yamai Sisters have been well known to the people who know about the existence of spirit because they often cause major storm in the area every time they come through to Earth.