This adorable piece was put together by Sleet777!
You can find more of his art posted on Twitter here! Check out Sleet 777 (sleet777): twitter.com/Sleet777 :3

It was another cloudy afternoon in the Sinnoh region, and it had just rained the previous day, so there was a lot of mud and puddles along the path of Route 201. Kai went to go see the guardian of Lake Verity, Mesprit, otherwise known as the Being of Emotion. He had a few chance encounters with this Pokémon before, and the more times they met, the better they got to know eachother. He knew that Mesprit lived in an ancient cave in a big lake, but he didn't know where it was. He asked many other Pokémon about its location and he was given some directions. At last he found the way to Lake Verity, and upon entering the large cavern, he was happily greeted by his budding friend, Mesprit, who was quite joyful to have visitors in her humble abode to break the mold after spending most of her time burning the days away in loneliness and boredom.

True to her legendary title, Mesprit was a very playful and expressive Pokémon, always looking for an excuse to come out of hiding and have fun, much like that of another mythical Pokémon, Mew. She rarely ever had a chance to make friends with the local Pokémon that lived nearby so the time she spent with Kai was very precious to her. 

She decided to play a game of tag with the Vaporeon, and immediately darted out of the cave, getting a head start as Kai followed behind, pursuing the vivacious fairy. They circled laps around Route 201 and different parts of Sinnoh afterward, having a wonderful time together and visiting the different landscapes along the way. Truly this was a memorable time to be had, most especially with a legendary Pokémon for a friend!