I feel like I should know who the woman in the hat is. Was she in the story previously or is she just a character the artist really liked the look of?
IIRC those 4 are the daughters of the previous Daba King, with the youngest sister being the current king along with the MC's middle sister.
Edit: Or maybe it's just her. IDK, I remember the king had a shitload of children and they all had to fight in order to become the next king.
If you're referring to the girl on pg.4 sitting on the chair cover illustration
She's Laurack, the "head honcho" madam of the brothel, Desert Spring, in the Daba Kingdom that you first see her introducing herself in ch.12 pg.19 when Sasae's group came to the brothel for info gathering
(and the mate of the eldest prince Jilq'End; well was)
After getting a taste of Sasae alongside two other fellow employees of hers (Laurack also fell for him too from that, which she mentions to Jilq'end during the siblings' fighting for the right to become king)
Fun Note:
The only one girl on the uh left (right side for us readers) of Laurack is probably the same employee from ch.12 pg.24-25, 31
The girl with her arms crossed is probably the same bodyguard in ch.12 pg.21