William

William is the deuteragonist of Diary of a Neapolitan Mastiff, who provides Danny with his first three diaries before Danny writes the rest by his own initiative.

Personality
William is responsible, loyal, practical, intelligent, courageous, elegant, and well-mannered in the TV show. He gets along well with the other Royal Guards and would do anything for them and the citizens of the Pocket Kingdom. In the book series, he is more relaxed and informal, and somewhat power mad as he briefly lets his position of ruler get to him in Ava's absense. He grows to be a strong and thoughtful, representing the Element of Thoughtfulness. He keeps Danny occupied by getting him diaries to write his experiences in, which begins the book series. He can also become furious, as seen when he attacks the Petbuster after he killed Ava by accident.

Book series
William is a Royal Guard, and meets Kate when she accidentally comes to Pocket Kingdom. William is left in charge of the Kingdom in the absense of Ava, and becomes somewhat power mad, cosntantly demoting Danny and taking advantage of his status. He eventually apologises and the two make up. When Ava returns, William is once again made second in command.

Three years later, Ava struggles to find the chosen dog in the pocket-pedia to be sent away to Earth. Matters are made worse when the Petbuster teleports to Pocketville to attempt to catch Ava, but accidentally kills her. William becomes distraught and he promises her in her final moments that he will hunt him down. William teams up with Danny, Magic, Pally and Wallis to take down the Petbuster, and return him to Pocketville. The friendship these five showed during their journey ignites the magic from their hearts to resurrect Ava, who in-turn kills the Petbuster and unleashes the magic across the Kingdom, allowing all residents of the land to become more open-minded, accepting, friendly and polite. Ava creates a purple friendship heart for herself, and allows William to keep the blue one.