Which Portuguese poet wrote under dozens of 'heteronyms'?

Fernando Pessoa invented something like seventy writers and then wrote as them. He was careful that they were not pseudonyms: a pseudonym is a name on your own work, and these had biographies, birth dates, horoscopes he cast himself, physical descriptions, politics, and above all distinct styles. They reviewed each other in print. They disagreed with each other about poetry. One wrote a preface to another's book.

Three carry most of the weight. Alberto Caeiro is the unschooled country poet the others treat as their master, who writes as though metaphysics were a kind of bad eyesight. Ricardo Reis is a classicist doctor turning out Horatian odes about accepting fate. Alvaro de Campos is a naval engineer who starts as a Whitman-struck futurist bellowing at machinery and ends exhausted. Then there is Bernardo Soares, whom Pessoa called a semi-heteronym - an assistant bookkeeper in Lisbon, close enough to Pessoa to be uncomfortable, and the author of The Book of Disquiet.

He published very little while alive. After his death in 1935 a trunk was opened containing something in the region of 25,000 unpublished pieces of paper, and editors have been working through it since. The Book of Disquiet was assembled from those fragments decades later and has no settled order - different editions are genuinely different books. The surname, which he did not choose, is Portuguese for person, from the Latin persona: a mask.