JoJo is a small Who (the smallest of all) who provides the voice needed to make the other animals hear Whoville and realize Horton was telling the truth.
Jojo is only given a distinct personality in the Blue Sky feature film adaption of Horton Hears a Who, and Seussical the Musical.
Blue Sky Film Edit
Jojo is the son of Ned McDodd and he is oldest child, as well as the only boy, among the Mayor's 97 children, and will more than likely be the future mayor of Whoville, because in Whoville tradition "the oldest is next in the mayoral line". JoJo is also the smallest Who of all.
JoJo is so worried that he'll say something to disappoint his father that he finds it best not to speak- ever. But he's anything but silent. He hears music in the tapping of an old keyboard, feels the beat in the rhythmic bounce of a basketball and finds harmony in wind whistling over a comb.
It is later revealed that JoJo takes nightly excursions to the Abandoned Star-Study Tower, and the one time we see him about to leave, his arms are filled with various odds and ends (including an umbrella that looks remarkably like that of the Cat in the Hat's), are to work on what is rather literally a giant instrument (called "the Symphonophone" on the soundtrack): he had filled the observatory with these castaway items and set up an elaborate system to produce music. JoJo uses his Symphonophone to add to the ruckus needed to alert the citizens of Nool to their existence, in addition to his signature cry: "YOPP!"
Seussical the Musical Edit
In Seussical the Musical, Jojo is the protagonist, and acts as an energetic young boy (or sometimes girl, depending on the production) with big ideas. He is curious, and often gets into trouble. He befriends Horton, who has similar problems to him.
In the film, JoJo's silent personality has created a unique appearance, JoJo has long, black hair, pale skin, brown/auburn eyes,and is adjourned in black-and-grey striped fur, giving him an emo-esque appearance. Despite being "the smallest" his sister Hedy is smaller than him
In the special, JoJo is blond, has blue eyes, and wears an all-blue outfit with a red bow at his neck.