This is my first fanfiction, so sorry if it isn't amazing!
I'm writing this and in this fanfiction the Legendary Pokémon can transform into human form so...yeah. The first few chapters are going to be about each legendary duo/trio.
The young, 10-year old Xerneas walked out into the clearing, staring at the dying flowers around her. The sun wasn't shining, but everywhere she walked light followed, as the flowers sprung to life as she stepped on the ground.. She looked up at the sky, staring at the stormy clouds, while thunder and lightning flashed. She looked back at her childhood friend Yveltal, who was falling behind. Xerneas sat down and transformed into her bipedal form, setting down a basket. She said to Yveltal, "This is perfect! Come on, this is the best place for a picnic!"
Yveltal smiled weakly, as he too transformed and sat down. As he settled down all the flowers and grass around him wilted away and died. He had always loved Xerneas, since they both entered the Hall of Origin as children. Yveltal watched as the love of his life unpacked the basket, getting out some sandwiches. Xerneas looked up, and looked at Yveltal deftly for a few seconds. "Well, come on! I can't do this all by myself!" as her naive, childish voice rang out through the forest. Yveltal smiled and gently pulled the basket away from Xerneas's hands.
As he finished unpacking, he looked up, surprised to see that Xerneas was gone. Looking around, he saw Xerneas prancing around, and picking flowers. He called out to Xerneas, "Come on! I've got it all ready! What are you doing?" Xerneas smiled and leaped back, pulling out the bunch of flowers she had picked. "Here! For you." She giggled as Yveltal took it, but she suddenly stopped when the roses lost their bright color, and died. Yveltal stared morosely at the flowers and watched as the petals fell off. "Why does everything I touch have to die?"
Xerneas stared painfully as she watched Yveltal. She felt tears well up in her eyes, because she knew that she would never be able to admit her feelings for Yveltal.
Zygarde watched as the two returned home. He didn't enjoy it, but he knew as the trio master of the two he had to watch over them and make sure that they were safe. He trotted home in his dog form, carefully trailing them and making sure that they didn't get lost. All of the legendary Pokémon were still children, so he didn't understand why Arceus imposed such strict rules. Like, why did he have power over Xerneas and Yveltal? Why couldn't they just take care of themselves? He understood why some of the other Pokémon had trio masters, like Groudon and Kyogre had Rayquaza. Groudon and Kyogre had an insane amount of power each, as they had helped shape the world but they were extremely immature and petty, invading each others domain every 10 seconds.
Zygarde could understand why they had Rayquaza, because Groudon and Kyogre needed babysitting. But Xerneas and Yveltal didn't need caretakers, the young godlings seemed to like each other and were mature enough not to piss each other off for fun.