Rudy Kangaroo is the son of Sour Kangaroo and a supporting character of Dr. Seuss' Horton Hears A Who!. But compared to the book and the musical called Seussical he appears more friendly in the movie.
In the book and musical, he is a minor antagonist who agrees with every word his mother said. In the movie however, he's nicer and has his own mind, but also slightly naive and fragile. He is also one of the few animals in the jungle that actually believes Horton. But he is also slightly timid and too afraid to speak up to his strict mother. However, when the animals roped and caged Horton and Kangaroo was about to destroy the clover with the dust speck on it and all the Whos within it, in a pot of boiling beezlenut oil, he grabbed it, because he heard something coming from it. When his mother ordered him to hand it back, he finally got the courage to tell her, "No, Mom". The animals of the jungle said they also heard something coming from it. Immediately after that, he gave it back to Horton.
The animals then isolated Kangaroo for tricking them, but Horton forgave her. Near the end, Rudy and the other major characters sing Can't Fight This Feeling and him and all the other animals return to the main part of the jungle.