Their complexity. For example, T. S. Garp has a wild and eventful life which makes him interesting.
The title characters' growth and development. Homer Wells in 'The Cider House Rules' starts as an innocent in the orphanage and then has to face the real world with all its moral gray areas. His evolution from a naïve boy to a man making tough decisions is what makes him an engaging title character.
Often they are in unique situations. Owen Meany is in a small town with big - city - like problems, and his role as a sort of moral compass is fascinating. His strange physical appearance and his strong sense of purpose add to his allure. His actions are often unexpected, like his prediction of his own death, which keeps the reader engaged and curious about him.