Navigating a character like Rei Hino for a fan story needs you to crack her shell a bit. She's got this intense spiritual dedication, but in the original show it often gets played for comic relief or just to oppose Usagi. If you're writing a serious scene, forget the tsundere shouting for a second. What's underneath? A girl who communes with fire and sees visions, who carries a family legacy of shrine service and probably feels incredibly lonely in that role. A compelling scene for her might hinge on that isolation—maybe she's performing a purification ritual alone at dawn, and the focus isn't on the magic effects, but on the weight of the incense in her hands, the silence of the empty shrine grounds, the quiet ache of knowing this duty separates her. The 'compelling' part comes from letting that stern exterior falter just enough for the reader to see the person beneath the uniform.
Romantic scenes, if that's your angle, need similar care. Throwing her into a generic passionate embrace ignores her character. Her approach to intimacy would be fierce but deeply reverent, I think. A touch might be hesitant first, then all at once, like jumping into a sacred fire. Maybe the conflict isn't about will-they-won't-they, but about how someone as controlled as Rei handles the terrifying vulnerability of actually wanting someone. Does she try to read their fortune first? Does she get angry at herself for the distraction? That internal clash between duty and desire is way more interesting than just describing two people kissing.