Posai is very distinctive amongst her siblings due to her small size and long pink hair. She lives at the Feel Better institute with Nizarah and Mitori, who are apparently very concerned for her security-- whether for Posai's safety or that of others is unknown. Posai does not speak often, and gives the appearance of being somewhat disengaged from the everyday world.
During her first appearance in the comic, Posai was seen hiding in a cupboard, where she was discovered by Nizarah. While apparently struggling against being taken away from her hiding place, Posai managed to knock Nizarah sufficiently off balance that he fell down the stairs still holding her. Whether this was intentional is debatable, and Posai, unlike Nizarah, appears to have suffered no significant injuries from the fall.
While Posai is assumed to be the youngest of the Kokirin siblings, her actual age is unknown, and despite Nizarah's hesitant assurances to Mitori that he has seen her "playing like we used to", Posai does not actually seem to display much childish behaviour, though when she does speak she is somewhat blunt and to the point. She also seems to possess a certain amount of knowledge-- or possibly instinct-- about the older aspects of Ui.