Dr. Cornelius is Caspian's tutor, brought in to oversee his education when Caspian's nurse was banished by Miraz. Dr. Cornelius is a half-Dwarf, and keeps his ancestry a secret in order to survive. To the Telmarines, who have never seen a real dwarf, he is only a small, fat schoolteacher with a long silver beard. He is also secretly a small-time practitioner in magic.
When Caspian begins to question Dr. Cornelius about the existence of the Old Narnia, he eventually reveals his true heritage to Caspian and begins to teach him of the old ways in hopes that when Caspian is king, he will be kinder to the surviving Narnians.
When it is apparent that Caspian will be murdered in order to clear the way for Miraz' newborn son to be king, Dr. Cornelius warns Caspian and gives him Susan's horn, a prized item that he had suffered long and endured many hardships to find.
After Caspian leaves, Dr. Cornelius eventually escapes the castle himself and is able to locate Caspian and his army in the woods. There, he gives Caspian and later the High King Peter advice and support.
When Caspian is crowed king, he makes Dr. Cornelius his lord chancellor. Of Dr. Cornelius' later life, not much is known.