Burst Heart

The Burst Heart is an item in the Pokémon réBURST manga that allows humans, known as Burst Warriors, to fuse with Pokémon.

Description
The Burst Heart is a small, pocket sized jewel that actually holds a Pokémon inside of it, sealed so that its power can be used by anyone wishing to fuse with it. The Burst Heart can be compared to a Poké Ball in that it holds a Pokémon inside, but is also different due to the fact that it doesn't store a Pokémon to be released for later use. If a Burst Heart is broken, the Pokémon inside will be freed. It is not known how a Burst Heart is made, but Artificial Burst Hearts (Japanese: 人工 Ｂハート) have been created by Great Gavel.

It is shown that a person can actually be sent inside of their Burst Heart in order to better connect with the Pokémon inside. In Ryouga's Burst Heart, an entire field can be seen inside and a large area where his Zekrom is being held. Once he arrives to this spot, a key takes the form of a young gatekeeper boy to test him and see if he's worthy enough of seeing Zekrom. Once Ryouga passes the test, the boy turns back into his true form—a key—and allows him to see Zekrom.