I like to think this story existed in the Pokemon universe as a myth that shows Yveltal in a neutral/positive light.

Characters: Giant (Lysandre), Death/Yveltal (X), Man (Trevor)

Back in the days when Yveltal was awake, he would often take in the form of a male human.

One day during that certain time, he encountered a giant of the name Lysandre when he was on Akala Island of the Alola region. For some strange reason, the mortal was able to recognize him.

"How could someone as small as you be as powerful as a god?" the giant asked.

"Because it's definitely a good idea to walk around in true form," Yveltal replied sarcastically. "That aside, it seems like being a male human garners the trust of many people and Pokemon. Don't know why but makes my job easier.

"Hmmmm..." The mortal then bend his head down to the deity. "I challenge you to a battle."

"Well, I guess I need something to get my mind out of," the god shrugged. "Those souls can wait." He then turned into a Lucario and rushed towards the giant while charging an Aura Sphere.

"Agh…" Yveltal rasped, as he panted and placed his knee down for a bit. He looked at Lysandre's back, still standing tall and sweat only trickling down his back, while Yveltal was tired to the bone. He leapt at the giant, and to his shock, Lysandre turned around and punched him in the face. Crashing to the ground, he winced at the crack of his skull.

"I win," Lysandre chuckled. To empathize his moment of glory or to taunt Yveltal, he stepped on the poor Pokemon, breaking even more bones.

"Owwwwww…" Yveltal moaned. He can feel the energy of the living rushing to him. "Oh no!OH NO!DON'T GO TO SLEEP YVEI! GOING TO SLEEP MEANS XERNEAS and LELE IS GOING TO KILL YOU!DON'T GO SLEEP!DON'TGOTOSLEEP!" But he felt uselessness in the struggle, as more energy flood into him, and sleep crept into his eyes….

Fortunately for the god, a young man named Trevor was walking by when he saw him.

He winced. "Gotten hurt badly hadn't you?" He took out a potion from his pocket and opened it. "Open your mouth."

The deity complied, as the potion was poured into him. In a moment, he felt better than before, and stood up.

"Thank you for your help," Yveltal said, then grimaced from the pain of his broken ribs. "Argh...maybe I should visit Xerneas for this later."

"You can talk!?" Trevor exclaimed.

"Of course I can," Death growled in pain. "I am Yveltal, in the form of a Lucario." He winced. "I had accepted a challenge from a giant and would've destroyed everything around here and get onto the wrong side of Xerneas and Tapu Lele if it wasn't for you."

"Well you're welcome!"

"As my thanks to you, I will send you messages that will warn you of my coming," Yveltal said. "Have a nice life." He limped away, grumbling words that Trevor can't hear.

Years past, and Trevor had many illness raid his body, but managed to lived through them. One day, while he was in the woods, Yveltal in Lucario form appeared in him.

"Uh Yveltal?" Trevor asked.

"Yes Trevor?"

"How come you appeared before me? You never told me you were coming."

The deity of death sighed. "So you never saw darkness even with your eyes opened? You never heard of things that shouldn't be there? You never felt like you want to eat even though you're starving? You never felt energy seeped away from you even though you did nothing? You never heard my children Cresselia's and Darkrai's warnings of me every night? You never ever had the desire to die?"

"Oh," Trevor realized. "Fine. Let's go."

"So I thought," Yveltal mumbled, then drained his life energy and took his soul to the afterlife.