The Cowardly Lion is the last straggler that Dorothy picks up on the yellow brick road. He makes a poor first impression by attacking the travelers—how rude! —but Dorothy immediately puts him in his place by bopping him on the nose. "You are nothing but a big coward," she tells him. "I know it," says the lion. "I've always known it. But how can I help it" (6.11-6.12)? Ha!
Almost immediately it's clear the Lion will be a friend, not a threat. He's wild at heart, but he knows how to live among polite society. And he becomes even more sympathetic as we come to understand how unhappy he is. Being a coward "is my great sorrow, and makes my life very unhappy," he tells Dorothy. "But whenever there is danger my heart begins to beat fast" (6.26). As a lion, he's supposed to be king of the forest, but he feels like a fraud.
Once the gang is back on the road, the Lion proves his bravery early and often. When they come to a huge ditch that's blocking their path, he jumps across with the travelers on his back. "I am terribly afraid of falling, myself," he tells them, "but I suppose there is nothing to do but try it" (7.14). Soon after that, when he and his friends are being chased by vicious beasts, he's prepared to make a final stand. "We are lost, for they will surely tear us to pieces with their sharp claws," he says. "But stand close behind me, and I will fight them as long as I am alive" (7.33). A pretty impressive act of bravery, no?
Still, the Lion lacks confidence until he receives a drink of courage from the Wizard of Oz. Though the drink is only a placebo, the Lion gets an instant dose of self-esteem, declaring that he feels "full of courage" (16.48). Finally, he feels more confident about what he's been doing all along. When he and his friends journey to see the Good Witch of the South, he defeats a giant spider and becomes the king of a forest. At the end of the book, he says, "If I could only get back to this forest I would pass my life very happily there" (23.25). Since Glinda plans to send him back there (via Winged Monkeys), we can only assume he's about to start living his best life.