Daniel is the protagonist of the novel (if you couldn't tell from the fact that his name is, uh, the title). When we first get to know Daniel, he's a brooding twenty-something at the casino, staring at Gwendolen as she gambles. But Daniel seems to have a lot to brood about. Think about how stressed out you'd be if you had absolutely no clue as to who you are or where you come from. Sir Hugo raises Daniel and puts him through school, but Daniel can't even definitively say whether he was adopted or if Sir Hugo is secretly his father. Bummer.
In spite of all of his worries, Daniel turns out to be a pretty great guy. He cares deeply for the people around him, even if he doesn't necessarily like them all the time. We learn that he has a tendency to care about people who are going through rough times. For example, when he's in college, Daniel blows his own chance at winning a scholarship competition because his friend Hans gets an irritation in his eye and can't study. So Daniel helps him with that, because he knows that Hans' family is poor and needs the scholarship money more. Another person Daniel helps out, but without necessarily setting out to do so, is Gwendolen. We're not entirely convinced how the two characters become so important to one another, but Daniel becomes a sort of advice-giver and sage to her, instructing her on how she should fix her selfish ways.
Perhaps the most life-changing experience of Daniel's life, though, is when he saves Mirah's life. He's just rowing out and about one day when he notices her looking miserable and preparing to drown herself. Daniel doesn't just rescue her; he finds her a place to live with his close friends, the Meyricks. He checks up on her. He hears her story and tries to find her family for her. What Daniel doesn't seem to expect is the way that Mirah inspires him to learn more about Judaism and to appreciate Jewish culture. As a result of his interest in Mirah's identity, Daniel starts studying Hebrew and strikes up a close relationship with Mordecai, who turns out to be Mirah's brother. Daniel's relationship with Mirah strikes pretty close to the heart of his search for his own identity. All of his curiosities about her family members make him reflect back on all the curiosities he has about his own secret background.
Though when we first meet Daniel he's about as confused about his place in the world as he can get, by the end of the novel it seems like everything falls into place. Daniel figures out his family's identity and finds out that he's Jewish. He realizes he's totally hot for Mirah and asks her to marry him. He even finds a purpose in life: when Mordecai dies, Daniel goes off to learn about the Jewish people of the world and aims to start a new Jewish nation. Daniel goes from being confused and unsure of his identity to having a clear background, a family, and a goal in life. Good job, Dan.