If this novel has one, single protagonist, it probably is Tom Joad. He's the first character that we meet, and we are deeply invested in his well-being. Tom turns out to be a fountain of knowledge, and he's the one who gets things done. He takes good care of his family, and he works tirelessly to help them survive. Reverend Casy helps Tom to understand the world and to find a higher purpose. Tom, who largely only been concerned about his family in the past, develops over the course of the novel to care for all of the downtrodden families he sees around him. He dedicates his life to fighting for justice and equality among the migrant worker community at the end of the novel. We love him because he's so helpful and so selfless. We are drawn to him because he grows so much throughout the course of the novel.
The Joads are our heroes, and at times it's hard not to think about them as a family unit. We want them to succeed and to prevail. We want them to find happiness in California. We root for them as they encounter obstacle after obstacle on their quest for peace. They are the focus of our attention.