When I saw this artwork, I immediately got a crazy, headcanon theory-story in my mind regarding Kaguya and Mokou's father (insert name).
After Kaguya had turned down Mokou's father's marriage proposal after he failed her trial of the Five Impossible Requests, he became sorrowful and depressed when he returned to his family.
Despite that his family's honor was shattered and became a low-spirited person, his love and care for Kaguya was still unflinching. So instead of proposing her for the second time, he decided to befriends with the Moon Princess, which she happily accepted.
Both Kaguya and Mokou's father became close friends for two months but he never returned to home during that period; his family members taught that he ran away out of despair due to his rejected proposal. It's during this time young Mokou learned about her father's rejected proposal and the ruination of her family's honor. As a result, Mokou aspired to recover her family's lost honor by taking revenge against Kaguya.
However, during the time when both Kaguya and Mokou's father were carrying out their friendly relationship, a man, who was one of the men who sought Kaguya's hand in marriage but also got rejected by her, suddenly came at them with a tanto in his hand and emotionally disturbed. The man became very angry and discontented after Kaguya turned down his proposal and he decided to kill her and then commits suicide afterwards.
When the angry man began charges towards them, Mokou's father put himself right in front Kaguya and took the stab in the abdomen that was meant for her. After the angry man had accidentally stabbed Mokou's father, he immediately took out a spare knife and stabbed Kaguya in the chest; not knowing that she was an immortal and couldn't die with that injury. Right after she was stabbed, she then fell to ground and pretended to be dead. The angry man then quickly fled from the place and jumped to a nearby cliff and fell to his death.
After the angry man died, Kaguya woke up and went to check on Mokou's father's condition. His injury was severe and he had lost a lot of blood. Despite that Kaguya revealed herself as an immortal and finally explained the reason why she turned down all those marriage proposal, Mokou's father assured her that he didn't care who she was; he told Kaguya that if both of them were married, he would do his best to give Kaguya a purposeful life and make sure that she would never feel alone and hurt after he had passed away.
After Mokou's father confessed his love for Kaguya for the second time, he finally succumb to his wound and died, leaving Kaguya full of guilt and sorrow towards him before she ran away and never to be seen again. Since that day, Mokou's father would remain in her memories and heart forever.
To further exacerbate the situation, the death of Mokou's father, which everyone mistook it as suicide, only further reinforced Mokou's desire of vengeance and deepened her hatred of Kaguya.