At the start, Joey is a pretty ordinary ten-year-old boy. He doesn't get along with Emily (big surprise), but he does get along with Allan, Jill, and Anna. Honestly, he kind of likes picking fights with Emily. While he gets along with his other sisters, that doesn't mean he's above teasing them...which he most definitely does. And Allan is quick to tell him to treat his sisters properly.
Joey is very impulsive, and rather lacking in common sense. He rushes into things without thinking, including rushing down a pitch black secret passage at the top of the house, not knowing at which point the stairs back down start.
He's not really much of a reader. He likes stories, but unless his Grampa is reading them aloud, he prefers them to be in a visual medium. Since he did listen to Grampa read, he is familiar with children's classics like the works of E. Nesbit, Rudyard Kipling, and of course C. S. Lewis. I don't think it ever actually comes into the story, but he likes Star Wars and is generally more of the sci-fi type. He would probably think the Watsons' adventures in Across the Stars were pretty cool. They are connected by mutual friends, after all. (Eventually, anyway. This is the year Sara was born.) He also is the type to think that getting captured by pirates is cool. I have a feeling that if he was from a few years later he would rather like Pirates of the Caribbean.
Joey originally wasn't the protagonist. But I think he's a good one. I was talking briefly about it with my sister one morning, and I realized that Joey is the better protagonist because he grows. He changes throughout the story. Jill is more of a static character, someone who influences the protagonist to change. Allan is the same. I'm glad Joey is one of my protagonists. (I would say Adriel is one too, and possibly Eleanor, but Adriel doesn't come in until book 2, which is also where Eleanor's story is told.) Joey's character voice is interesting, and he is a fun character to write.