“Around 1912, if I’m not mistaken (not greatly anyway), the idea came to me to write poems of a pagan nature. So I scribbled something down in irregular verse (different to the style of Álvaro de Campos, more irregular), and abandoned the idea. It was a badly woven twilight, a blurred portrait of the person who was composing it. (I hadn’t realized it yet, but that was when Ricardo Reis was born).”
Fernando Pessoa writes in his letter dated Janeiro 13th 1935 to Adolfo Casais Monteiro, that Ricardo Reis was born in 1887 (although he couldn’t recall the exact date), in Oporto. He describes him as shorter, stronger and stiffer than Caeiro, besides being clean shaven. He had had a Jesuit school education, was a doctor and had lived in Brazil since 1919, from where he had been self-expatriated for being a supporter of the monarchy. He had Latin and semi-Hellenic instruction.
Fernando Pessoa admits he conferred to this heteronym and excessive purity and writing as Ricardo Reis mentions he “followed an abstract deliberation which immediately took the shape of an ode”.

Source: Fernando Pessoa’s Letter to Adolfo Casais Monteiro, January 13th 1935, in Correspondência 1923-1935, ed. Manuela Parreira da Silva, Lisbon Assírio & Alvim, 1999.