You are a trainer, one whose first partner was an adorable little Ralts, whom you loved very much. To you, your partner was more than just a Pokemon for competing or a pet; they were more like your family, like a child you raised. Which made it interesting raising this particular Ralts.
From early on, you noticed something strange about this Ralts. It seemed to always be hungry, and loved sucking or nibbling on things, including you. One day, when it had wandered away from you in the forest, you stumbled upon it with a Caterpie's tail sticking out of its mouth, soon gulped all the way down into the Ralts's stomach. From that day forward, every now and then your Ralts would find some excuse to catch a Pokemon smaller than it and swallow them whole. And... while you found this disturbing... you couldn't bring yourself to scold Ralts.
When Ralts evolved into Kirlia, the adorably shy little thing became a lot bolder, and just a bit bratty. And with this change in personality came an increase in hunger. The little Kirlia, who by this point nearly always had a bit of pudge from its diet, was now looking for live meals every day, refusing to eat anything else if it hadn't already gulped something squirming down its. On top of that, Kirlia began ingesting things with other body parts as well, seeming to get a thrill out of it. And finally, you noticed it starting to eat prey larger than itself, like when Kirlia slurped an entire Ekans into its belly.
Finally, recently, your Kirlia evolved into a beautiful and powerful Gardevoir... and while your partner has matured and is no long quite the brat she was before, you can tell that its hunger has only grown with evolution. You can tell Gardevoir loves you... but you've also got the feeling that you yourself are on the menu, given the looks it's been giving you. You can tell it's planning on eating a human, any human; all your partner's life, you've noticed them giving humans hungry looks or sampling your flavor whenever she cuddled with you.
Today, after setting up camp, you catch it staring at you, hear the Gardevoir's stomach growling loudly as you realize it hadn't eaten anything today... and then it begins to approach you.