The relationship between Demon King Naruto and Hinata could start off rocky. Naruto might be worried that his new powers and the dark energy associated with being the Demon King could harm Hinata. However, Hinata is determined to stay by his side. As they face various challenges together, like battles against those who oppose Naruto's new existence or dealing with the internal struggles Naruto has with his demonic side, their relationship evolves. Hinata helps Naruto see the good in himself even in his Demon King form, and Naruto in turn becomes more open and affectionate towards Hinata, leading to a relationship that is both passionate and supportive.
In a fanfiction, at first, Naruto as the Demon King might be afraid that his new form will scare Hinata away. But Hinata, with her unwavering love, shows him that she accepts him regardless. As time passes, Naruto starts to rely on Hinata more and more. They share intimate moments where Naruto reveals his fears and insecurities about his new identity to Hinata, and she offers him comfort and support. Their relationship grows from a simple love into a deep, unbreakable bond that is based on trust, acceptance, and mutual understanding.