Gardevoir's loyalty and devotion makes sense from an evolutionary (Darwinian that is) standpoint if you look at its pre-evolved forms. Ralts is one of the weakest Pokemon in the series, even outdoing Magikarp and Kirlia is the weakest evolved Pokemon that isn't a sedentary cocoon. Gardevoir evolved to fiercely protect those it loved, their children, as they were so vulnerable.
Fridge: Pokémon Ruby and Sapphire, TV Tropes

Throughout the Great Sea, there echoes a legend...
A legend of a hero who traveled through time to save a kingdom where a golden power lay hidden from an evil tyrant. He became known as the Hero of Time, and the inhabitants of that land came to rely on him. But when the world needed him again, he did not appear, despite the countless pleas to the gods.

What became of that kingdom? None remain who know.

But still, the legend continues on the wind...

When I was just a little Ralts, I spent a lot of time wondering about a lot of things. I used to wonder what lay beyond the endless ocean we called home. I used to wonder if I'd ever do anything great. I used to wonder if all of the stories we were told were true, the legends of ancient heroes. And then, one day, unfortunate circumstances caused me to go out on a journey which ultimately lead me to discover all sorts of truths, even the truths of things that I'd never even wondered about. I'd even go as far to say that I discovered things mortals should never have known...
Okay, maybe that was a bit over-dramatic.

Arc-en-Ciel suggested that I wrote down my story, so that it would never be forgotten. Who knows, maybe future generations will find this incredibly interesting.
My name's Link, and this is the story of how I discovered the secret of the Great Sea.

"Hey brother! Wake up!"

It was another morning on the peaceful little Outset Island, where a few Ralts, Kirlia and Gardevoir (but not Gallade, because Dawn Stones were very rare and expensive), such as myself and my family, lived. The sky was clear, and I could hear the sounds of the calm sea and Wingull. I woke up to find myself asleep on the watchtower, and my sister Aryll shouting at me. Being tired, I wasn't sure why she was so excited.

"Guess what today is!" she exclaimed.
I hadn't really woken up yet, so I just stared.
"Happy birthday, big brother!"
"Oh!" How had I forgotten it was my birthday? I had barely gotten any sleep the night before, which must've been how I ended up on the watchtower.
"Link! Grandma would like to see you!"
Aryll and I lived with our grandmother. Our parents died when my sister and I were only young. Grandma was kind, but I sometimes worried about her, having to manage with two children on her own.

I made my way down to the village. There was an upside-down triangle symbol on all of the doors in the village. The townsfolk were all willing to talk to me, wishing me a happy birthday and telling me useful information. I eventually made it to my house.

"Happy birthday, Link!" said Grandma. "Today, you're the same age as the hero from the legends. You're also around the age where, a long time ago, young Ralts were taught the ways of swordplay. But now we live in an era of peace, and we have no need for that anymore."

She then told me that there would be a big party in the evening, but it would be a surprise, and that I should talk to Aryll in the meantime.

I eventually made my way back to the watchtower, where many Wingull were gathering. Aryll was waiting for me there.

"Close your eyes and hold out your hand, big brother!" she happily exclaimed.
I did as instructed, and Aryll gave me… her telescope? She never let go of that thing.
"You can borrow it for just one day!"

I used the telescope, and saw that the Fletchinder postman, Quill, was delivering letters to our house. Suddenly, though, he looked as if he was startled by something.
"Look! Over there!" Aryll shouted.

I moved the telescope upwards, and saw that there was a giant, brown winged Pokémon – I think it was called a Staraptor – wearing some kind of mask. Upon closer inspection, it was carrying a Kirlia in its talons. The Staraptor was then hit by a rock, dropping the Kirlia into the Forest of Flabébé on the top of the island. I couldn't just leave someone in need on their own; I had to do something.

"Aryll, I'm sorry, but I'm going to have to rescue her," I said. "Don't worry about me, okay?"

Before I could go to the forest, I needed to get a sword, because the path was blocked by some small trees. The only swordsmon on the island was Mantine (who wasn't actually a Mantine). The Pokémon of Outset Island rarely had to defend themselves, so most of them didn't know moves, and when they did, they were usually defensive ones. We mostly resorted to using weapons. Everyone on the island knew each other well, so I entered his house.

"What is the matter, Link? You look as if something terrible has happened," he said.
"Well, a giant Star-" I tried to respond, but was interrupted.
"Whatever happened, I suppose it was no laughing matter. Now, would you like to learn the ways of the sword?"
"Umm… yes."

And so we began my sword training. He first taught me how to do a horizontal slice, which involved me attacking him. I was a bit nervous about it at first, but then when I actually attacked the old Gardevoir, he blocked it. I attacked him with the horizontal slice a few more times, and then he taught me the vertical slice, and the trust, and the spin attack (which made me rather dizzy), and the parry, and the jump attack, repeating the process every time.

"That was a fine display!" said Mantine, once we'd finished our training. "And yet… I sense a certain anxiousness in the way you use your sword."
"That's because I'm going to resc-" I tried to say, but was once again interrupted.
"I do not know what has happened, but I have faith in you. Take that sword with you, and use it well."

And so I climbed up the slope to the Forest of Flabébé to rescue the Kirlia girl. Little did I know that this was the start of something much greater...

Before you ask why Mantine/Orca is male and a Gardevoir: Gardevoir can be either gender. If you don't believe me, try getting a male Kirlia to level 30 in a game of your choice.