The plot could be that Serena is exploring a new region when she stumbles upon an old, abandoned castle. Inside, she finds a Pokemon that is trapped in a form that makes it look like a beast. This Pokemon was cursed long ago. Serena, being curious and kind, starts to interact with it. She brings beauty into its dark and lonely world. As they get to know each other, the Pokemon starts to change, becoming less beast - like. Their relationship evolves, and Serena has to face challenges from those who don't understand her connection with this strange Pokemon.
A possible plot is that Serena gets separated from her friends during a journey. She then finds herself in a place filled with mystery. There, she meets a Pokemon that has a very intimidating appearance, much like the Beast. This Pokemon is initially hostile towards her, but Serena's gentle nature and her beauty (not just physical but also in her actions) start to have an effect on it. She tries to communicate with it, and gradually, they form a bond. The Pokemon begins to open up to her, and together they embark on a journey to break the curse that binds the Pokemon, similar to how Belle and the Beast's story unfolds.