Francisco Marto was nine in the summer of 1917, and he is the least documented of the three children, partly because of what the account itself says about his part in it.
What he reported
By the account the three of them gave, Francisco saw the figure at the Cova da Iria but did not hear her speak. Whatever was said reached him through his cousin Lucia dos Santos, and sometimes through his younger sister Jacinta, and was repeated to him afterwards. He is the one member of the group who took the whole content of the apparitions on somebody else's word.
What he did about it
The devotional literature is consistent that his response was to go off on his own and stay in the church at Fatima, and that where the other two spoke of converting sinners he spoke of consoling God. Whether the piety of a nine-year-old can be reported accurately by adults who already believed the apparitions is a fair question, and the answer is that most of what is known about him was written down after he was dead, by people who did.
The arrest
In August 1917 the administrator of the district, an official of an aggressively anticlerical republic, took the three children from their homes and held them. By every account, including hostile ones, none of them changed the story under it. Francisco was nine years old.
The end
He fell ill in the influenza pandemic and died at home in Aljustrel on 4 April 1919, ten years old, having said for some time that he expected to die.