Yume (夢, Yume-chan) is the technical owner of Maxwell. He is owned by her older brother, but she's the one that actually looks after him. She is also the youngest child of the book store owners.
In the episode A Little Love Story!, She writes a book about a hamster called Nana and her love for a hamster named Yu. This interestingly takes the same form of relationship Maxwell (her hamster) had with Sandy.
In The Ham-Ham Explorers and the Romance Story!, she makes a second book, this time of hamsters on an adventure in a cave. Once again the plot mimics that of the Ham-Ham's adventures.