Wicked: The Life and Times of the Wicked Witch of the West
Dr. Dillamond, our favorite Goat, is the prime representative of Animals in the book. He gets very little air time, so to speak, but he's a huge influence in Elphaba's life. He's actually a bit like Fiyero in that regard: he has a massive impact on her adult life without spending that much time with her.
Dr. Dillamond is primarily characterized by his fight against the Wizard's unjust treatment of Animals and his quest to prove that Animals are equal to humans. His passion for this quest draws in Elphaba (and Boq, Crope, and Tibbett), but we almost never see Elphaba interacting with her mentor.
This quest, and Dillamond's inspiring passion, continues to drive Elphaba for the rest of her life. Dillamond's murder also teaches her a lot about evil, the real world, and the fact that Madame Morrible is crazy villain. (Well, she may have already been aware of the last point, but Dillamond's death hammered it home.)
What's really fascinating is that we never actually get a conversation between Elphaba and Dillamond; we only hear about him second-hand from her. Elphaba sort of functions as Dillamond's heir apparent (she's using his books in her monkey surgery near the end), so in mainly hearing about Dillamond through her, it's like the transfer of power is already taking place. Dillamond definitely has a lasting legacy, and he influences the lives of not only Elphaba but Galinda/Glinda and Boq as well. Unfortunately for Dillamond, the bulk of his impact on the novel comes out of his murder.