Eva

Eva is the twin sister of Princess Ava in Diary of a Neapolitan Mastiff.

She is jealous of Ava's position of power and conspires with her two dog guards Soul and Gourt to steal the Friendship Heart to overthrow her.

Personality
Eva is intelligient, clever, devious, but jealous and cruel, attempting to steal her sister's power and ripping up Danny's personal diary.

Book series
Just like in the TV Show Adventures in Pocketville, Eva's attempt to snatch the Friendship Heart during Magic's (then Oristalfo) friendship ceremony ends with the heart breaking in two and Ava sent to Earth. Eva recovers one half while Kate, brought to Pocketville from Earth in Ava's place, recovers the other half, using it to communicate with the dogs and cats. Eva sometime later tears up Danny's first diary. Danny speculated that Eva began the death curse although later believes this wasn't the case. Eva's adventures continue "off-screen" as they do in the show, not being portrayed, until she commands the Petbuster to steal the other half of the Friendship Heart. The Petbuster fails and he is jailed, and Eva gives up her half of the Friendship Heart, allowing Kate to restore the whole jewel so she can later steal the whole thing when they perform their ceremony to restore Ava to Pocket Kingdom. However, as Eva leaps to the restored heart, she and Ava switch places, and the others note her as missing. In the show, Eva was then captured by the released Petbuster, however this hasn't been explicitly stated as canon in the book series, though it can be safely assumed. Pocket Kingdom note her as missing for three years until she sends a voice message to Ava which says "I'm back", after war is declared against the Zecorans. Ava has a gut feeling that Eva has stolen the Black Heart, the opposite of the Friendship Heart used to channel dark magic. Ava and Danny race to the chest it is kept in within the castle, and indeed find it stolen, presumably by Eva.