Lucy Burroughs answered
The Chronicles of Narnia series by C. S. Lewis spans several decades and locations, and as a consequence, contains a multitude of characters. The focus tends to shift from book to book, but there are several recurring characters, including Aslan the lion, who - if you had to pick just one - would be the main character, and is the only one to appear in all seven novels.
The Magician's Nephew - The main characters in this book are two pre-teen friends, Digory Kirke and Polly Plumber. The antagonist is Jadis, who later becomes the White Witch. This book shows the beginning of the world of Narnia.
The Lion, the Witch and the Wardrobe - Aslan and the four Pevensie children are the central characters in this novel. The children, from oldest to youngest, are Peter, Susan, Edmund, and Lucy. The antagonist is the White Witch (Jadis from The Magician's Nephew).
The Horse and His Boy - Shasta, an uneducated boy adopted by a fisherman, is the central character. He later turns out to be Prince Cor, the long-lost twin brother of Prince Corin and heir to the throne. Prince Rabadash, obsessed with the idea of marrying an unwilling Susan, is the antagonist in this book. Shasta/Cor's talking horse Bree also features throughout.
Prince Caspian - As the title suggests, Prince Caspian (the rightful king of Narnia) is the main character of this book. Only a year has passed in England since the events of The Lion, the Witch and the Wardrobe, but in Narnia, several centuries have passed. Aslan appears frequently throughout.
The Voyage of the Dawn Treader - Lucy and Edmund, the youngest of the Pevensie children, return to Narnia with their cousin Eustace Scrubb. Prince Caspian is also one of the main characters, although he's now King Caspian X.
The Silver Chair - Eustace Scrubb from The Voyage of the Dawn Treader is one of the main characters in this book, too. He returns to Narnia with his school friend Jill Pole. The now-elderly King Caspian also features, along with his heir, Prince Rilian, who is captured by the evil ruler of the Underworld, the Lady of the Green Kirtle.
The Last Battle - This book shows the end of Narnia. It is revealed that Peter, Edmund and Lucy have died in a train crash with their parents, yet Susan is still alive in our world (and doesn't return to Narnia, as she's stopped believing). Aslan takes the children to Aslan's Country (as the series is a Christian allegory, Aslan's Country represents heaven) where they are reunited with their friends - the good characters - from the previous novels.