Sylph is a Gardevoir owned by Xander Cruize.
Sylph was born in a small community of Gardevoir and Gallade that resided on Kalos Route 4, south of Lumiose City. One day, while out exploring the area, she found herself separated from the others with no idea where she was.
As she was reaching out to call for her mother, another creature appeared before her. A Mightyena who, seeing easy prey, struck quickly. The dark type knocked Sylph to the ground and clamped down on her left arm, which she had raised to try and defend herself. With a quick motion, her attacker tore her arm off, just a bit below the elbow.
As she lay there screaming and bleeding, a young boy came to her rescue. He threw a rock that distracted the MIghtyena long enough for Professor Sycamore to arrive on-scene and scare it away. Seeing her injured state, the Professor quickly sealed her in a Pokéball and rushed to a Pokémon Center for emergency medical attention.
While recovering, Sylph came to know Xander and the Professor. She learned that Xander was a junior trainer whose goal in life was to help everyone he could. During her time in the Center, Professor Sycamore had a discussion with her father, and both parties agreed it would be better if she stayed in captivity.
A month after the incident, Sylph was taken to Sycamore's lab in Kalos. She was asked to demonstrate attacks and battle techniques, and was often partnered to train with Xander. When he turned 10, he took her as his starter Pokémon.
The two battled took on the Kalos Gym Challenge, taking a little under three years to clear all eight gyms and challenge the Elite Four. They defeated Elite Four member Drasna and technically defeated Wirkstrom before Sylph's injuries got the better of her. Despite this, they only claim victory over the dragon-type trainer.
Afterwards, the duo had two friendly battles with Calem and Serena, both of which they lost due to the champions' advanced training. Seeing that they needed to get stronger, Xander and Sylph retreated to the forest that made up Route 20 and began to train in isolation. During this time, Sylph and Xander developed a romance, and she began unlocking her trainer's psychic abilities.
After three years of training, the two reemerged and visited old friends before leaving Kalos for Hoenn, receiving a Fennekin and an Eevee from Sycamore as a parting gift.
Sylph looks like a standard Gardevoir except for the fact that she is missing her left arm starting just slightly below the elbow.
Sylph is somewhat of a shrinking violet, nervous in large groups of strangers. However, when she opens up to people, she is much more open with her dry, sarcastic humor. She is fiercely loyal and protective of her friends, and will mother those with whom she is especially close.
Sylph, being a Gardevoir, has access to several attacks. She is highly proficient with special attacks, and can use the remains of her left arm to launch uncontrolled attacks that cover a larger area and deal extra damage. This ability, however, takes extra stamina as she cannot inhibit the amount of energy flowing through the arm.
Sylph also possesses the capability of human speech, taught to her by her trainer over the course of several years.
Once this over, we must really talk to Tierra about where he tosses his garbage. It's having a negative impact on the environment. - Sylph upon meeting Feral Tierra
"That idiot is playing the hero again!" - Sylph, talking about Xander.