Less well known, I think, is that he was a deeply religious man and a fearsome ascetic. It is not a coincidence that he joined the Discalced Carmelites -- among the more mystical, contemplative, and historically, among the more severely austere of the Catholic mendicant religious orders (it was founded by Teresa of Avila and John of the Cross).
He almost never wore the traditional habit of the Carmelites (or even "the blacks" of a priest), because he felt clerical garb no longer represented what it originally was (the dress of the poor). He owned almost nothing, gave anything given to him to the poor (except books!), dressed like a plumber, and slept on the floor. I read somewhere that his office had a dictionary, a telephone, and not much else.
He was so famous for being "the Pope's Latinist." I have often wondered if anyone ever asked him about his faith or the pursuit of holiness. Because I sincerely think he took these matters just as seriously, and probably more seriously than Latin.