So, in short, the dark-haired, red-eyed man wasn’t unfaithful to the real Tareya—he used that fake lover to gain political power, but he never had a romantic relationship with her. He pretended to care for her in public to protect her from the guys who wanted to kill her because of her special power—since she could heal the bastard prince—and so he could use him in the future to stage a coup.
So, in short, the dark-haired, red-eyed man wasn’t unfaithful to the real Tareya—he used that fake lover to gain political power, but he never had a romantic relationship with her. He pretended to care for her in public to protect her from the guys who wanted to kill her because of her special power—since she could heal the bastard prince—and so he could use him in the future to stage a coup.