The official Pokémon movie website has updated with new story details and character profiles. Translations follow:

Story

One hundred years ago, the desert city "Désser City" got severely damaged in a battle between Pokémon. The battle was started due to one specific Pokémon.
However, the Pokémon got sealed by someone, and the city went back to being as peaceful as ever, a state it still is in today.

As Satoshi, Pikachu and the others continue their journey, they encounter the mythical Pokémon Hoopa, who carries "rings that can summon anything and everything", and a girl named Mary who takes care of Hoopa.

The mischievous Hoopa starts summoning all kinds of things from its rings, including its favorite food, donuts, to everyone's surprise and enjoyment.
Ever a prankster, the simple-minded and innocent Hoopa starts referring to Satoshi and Pikachu as "Saton" and "Pikan". It quickly befriends the group, and they all decid to travel together.

Once they arrive in Désser City, Satoshi and the others meet Mary's older brother, Barza.
However, something seems off about him. On a closer look, he's holding the "Prison Bottle" that's said to have sealed the Pokémon that once destroyed the city!

An evil air drifts out of the bottle. It appears that the seal has become undone over all this time.
Just then, legendary Pokémon giving off that same air appear! Primal Groudon, Primal Kyogre, Dialga, Palkia, Giratina and Kyurem... This will put the city in extreme danger.

In order to protect the city, Hoopa makes more legendary Pokémon "Appear!" from its rings! Will Satoshi and his friends manage to protect Désser City alongside the legendary Pokémon Hoopa made "Appear!"? And what is the true identity of this evil power?

LEGEND VS LEGEND!
The most fierce battle in Pokémon movie history is about to begin!

Character Profiles

Mythical Pokémon:

Picture	Biography
	Hoopa: By speaking the word "Appear!", it is able to summon all kinds of things, Pokémon and people included, with its rings! It loves pranks and donuts! Species: Mischief Pokémon Type: Psychic/Ghost Height: 0.5 m Weight: 9.0 kg Ability: Magician
	Arceus: A one-of-a-kind Pokémon said to have created the world! Species: Creation Pokémon Type: Normal Height: 3.2 m Weight: 320.0 kg Ability: Multitype

Legendary Pokémon:

Picture	Biography
	Latias: Species: Eon Pokémon Type: Dragon/Psychic Height: 1.4 m Weight: 40.0 kg Ability: Levitate
	Giratina (Another Forme): Species: Renegade Pokémon Type: Ghost/Dragon Height: 4.5 m Weight: 750.0 kg Ability: Pressure
	Lugia: Species: Diving Pokémon Type: Psychic/Flying Height: 5.2 m Weight: 216.0 kg Ability: Pressure
	Dialga: Species: Time Pokémon Type: Steel/Dragon Height: 5.4 m Weight: 683.0 kg Ability: Pressure
	Black Mega Rayquaza: Species: Sky High Pokémon Type: Dragon/Flying Height: 10.8 m Weight: 392.0 kg Ability: Delta Stream
	Kyurem: Species: Boundary Pokémon Type: Dragon/Ice Height: 3.0 m Weight: 325.0 kg Ability: Pressure
	Palkia: Species: Space Pokémon Type: Water/Dragon Height: 4.2 m Weight: 336.0 kg Ability: Pressure
	Primal Groudon: Species: Continent Pokémon Type: Ground/Fire Height: 5.0 m Weight: 999.7 kg Ability: Desolate Land
	Latios: Species: Eon Pokémon Type: Dragon/Psychic Height: 2.0 m Weight: 60.0 kg Ability: Levitate
	Primal Kyogre: Species: Sea Basin Pokémon Type: Water Height: 9.8 m Weight: 430.0 kg Ability: Primordial Sea

Characters:

Picture	Biography
	Mary (メアリ): A girl taking care of the mythical Pokémon Hoopa. She used to live together with Hoopa in Arkhē Valley. Barza's younger sister. She encounters Satoshi and the others in Désser City.
	Barza (バルザ): A young man who used to live together with Hoopa in Arkhē Valley. Mary's older brother. He found the "Prison Bottle" in the desert and took it with him.
	The Traveler (旅人): The person who sealed the Pokémon that caused great damage to Désser City in the "Prison Bottle" one hundred years ago.
	Serena: A girl from Asame Town who loves to dress well. She competes in Tripokalons while traveling around with Satoshi and the others! Her dream is to win the Tripokalons and become Kalos Queen!
	Satoshi: A Pokémon Trainer from Masara Town. Together with his partner Pikachu, he has come to the Kalos region to pursue his dream of becoming a "Pokémon Master". He loves Pokémon more than anyone else, has a strong heart and never gives up.
	Citron: He is from Miare City. He is the Gym Leader of the Miare Gym and uses Electric types. A talented inventor, his backpack is equipped with an Eipam Arm he is very proud of.
	Eureka: She is from Miare City. Citron's little sister. A girl with a strong character who is always looking for brides for Citron. Citron lets her "keep" his Pokémon until she becomes a Pokémon Trainer.

Sources: http://www.pokemon-movie.jp/story/and http://www.pokemon-movie.jp/chara/