The friendship heart is a jewel that is used to send pets from Pocketville to the Big City via the Magic Fountain. It was originally in the possession of Princess Ava, but has shattered into two halves. The halves are know in possession of Eva and Kate. Eva is trying to get the other half off Kate. It also allows for humans like Kate to understand Magic and the other pets of Pocketville. It's magic abilities allow for Kate and Magic to travel from the Big City to Pocketville and vice versa.
The words are from the Big City to Pocketville and vice versa are -
Kate - "My puppy and I together forever that's all that I want."
Magic - "Katie and I together forever that's all that I want."
- It's revealed by Ava herself that she made an oath to the jewel's power in Friendship is a Treasure.
- In Kate's Birthday the Heart is said to glow when Princess Ava is near.