As a person of half-dwarfish, half-human blood posing as a human, he was one of the few people in the Telmarine-ruled society of Narnia under King Miraz that knew of the history of the Old Narnians, and introduced Caspian to them.In the 2290's, after the death of King Caspian IX, he was chosen to serve as private tutor to the young Prince Caspian X. The two became very close as the Doctor gained the prince's respect and taught him many subjects, including mathematics, grammar, history, politics, and astronomy. Despite their closeness, Cornelius waited some time to reveal to the prince the true political nature of his uncle's realm. After some time, he subtly dropped hints of the existence of Old Narnia. When the young prince responded positively, Cornelius arranged for a private astronomy lesson between the two.
The lesson involved the collision of the planets of Tarva and Alambil an event Cornelius knew to be of fortuitous significance for the realm of Narnia. During the lesson he revealed his true dwarvish ancestry to the young prince, and confirmed the prince's belief in Old Narnia. That night cemented a lifelong friendship, and in many secret lessons following, Cornelius imparted much of his forbidden knowledge of Old Narnia to Caspian, who eagerly took in the studies, never telling anyone of their lessons. During this time Cornelius also advised Caspian to be a better king than his ancestors and to be fair to all peoples.
The era was brought to an abrupt end with the start of the War of Deliverance. In 2303, the Lord Regent, Caspian's uncle Miraz gave birth to his own heir. On the night of the birth, Cornelius crept into Caspian's room, setting a charmed sleep on the prince's gentlemen-in-waiting. Rousing the prince, he brought him again to the secret tower and told him many of the truth of his father's murder and his rightful place as king. He begged Caspian to flee for his life, giving to him his most precious gift, the horn of Queen Susan.
Happy hunting.