Years ago, you started your quest to become the Champion of Unova. You chose Snivy, the cute but proud Grass Snake Pokemon. The little serpent was quite proud but became VERY fond of you. She would coil around you, intending to look affectionate but coming across as possessive. But you understood her intention.
She quickly found herself the backbone of your team. She got a bit spoiled, of course, due to the bond you shared and her rather proud personality. She demanded your attention, and her teammates noticed, jockeying for your favor by working harder.
Your team excelled, but as she Evolved into a Servine, then a Serperior, she became ever more proud and demanding.
Then you became Champion. Suddenly, your dream was fulfilled. Now there wasn't anything special to do but fend off challengers, and when a bright young kid beat you, you had little need for another career. Your Pokemon had all been given only the finest, and now that you lost, you finally decided to retire.
You decided to get a nice new house for you and your team, while the others you caught were pampered on a ranch. You gave each of them their own nice, comfortable room, with your precious Serperior getting a large jungle-themed room with a large skylight in the ceiling.
Being used to getting anything she wanted, this was something she pretended she expected of you, but you caught the happy sparkle in those regal eyes.

Years later, you bring the snake's dinner in. She's resting her beautifully sleek, perfectly groomed form on a collection of soft pillows. Some were stolen from your room, but you were wealthy enough to afford such minor luxuries as pillows.
The Serperior raises her head, watching you closely and trying not to drool. She loved food. She spent most of her time lounging around in the sun. You place the meal in front of her, and she sniffs it. Of course, she was only pretending to be a picky eater. As she starts devouring her meal, you pick up the grooming supplies and get to work. Running a brush over her soft body, you note she's fatter than before. Of course, being the pampered ace of a former Champion, it was expected you'd spoil her. As you run a hand over her coils, she grunts and rolls slightly, showing her belly. It was smooth and soft. Her fat piled on in a unique way, almost evenly throughout her cushiony form. As you pet her middle, she drowsily sighs, utterly content with her life.
Her body was soft enough you mentally debate whether or not to use her as a bed. Then her eyes lock with yours. Unlike when you first met, this wasn't an arrogant glare, but a gentle invitation made clearer as such by the way she spreads her arms for a hug.
You make up your mind. If she wanted to cuddle, you might as well. As you settle onto the pillows forming your Pokemon's bed, you become aware of just how cushy you've made her life. She shifts her considerable bulk, revealing just how fat she's gotten. Then she subtly rests her body on you, then begins to wrap around you. Her body's soft and warm. You can hear her stomach gurgling softly in digestion. She steadily relaxes, and you hear her snoring lazily. A few minutes later you fall asleep too, perfectly comfortable in you precious Serperior's embrace.