The official Pokémon movie website has updated with information on the movie's theme song. Translations follow:

The Performer of the Theme Song for Pokémon the Movie XY: "The Archdjinni of Rings - Hoopa" Will be Rei Yasuda

The Performer of the Theme Song for Pokémon the Movie XY: "The Archdjinni of Rings - Hoopa" has been announced!
It will be performed by Rei Yasuda (安田レイ)!
Yasuda is showing signs of becoming a major breakout artist, her 5 stop solo tour through Japan that started in January 2015 having sold completely out at four venues!

The theme song's title is "Tweedia"!
Tweedia is the name of a flower, and in the language of flowers, it means "trust and blissful love"! Both the lyrics and the music carry the sense of a magnificent and warm atmosphere that fits Pokémon to a T!

Pokémon has been a part of Rei Yasuda's life ever since she was little, and she is a perfect example of "PokéWomen: Young women who keep loving Pokémon even into adulthood".
PokéWomen, make sure to pay attention to the synchronization of Rei Yasuda's singing voice and the Pokémon movie!

A personal comment from Rei Yasuda

When I was a kid, me and my brother looked forward to Pokémon on TV every week. Our house was full of cards and toys. I've been watching the Pokémon movies since back then, so it makes me extremely happy to be chosen to do one of their theme songs! Pokémon has reminded me of the importance of bonds and companionship many times. I'm happy that I'll get to build excitement for such a story of companionship and bonding with "Tweedia", which translates to "trust" and "blissful love"! I want for you to listen to Tweedia while picturing your dear friends and the people you know will always support you.

安田レイ YASUDA REI OFFICIAL WEBSITE

Source: http://www.pokemon-movie.jp/news/?p=1659