Elena goes to Ischia during the summer...
In Elena Ferrante's My Brilliant Friend, Donato Sarratore is Nino Sarratore's father. Donato and Nino have a difficult relationship: Nino does not like his father. This is referenced most specifically during the scenes in the book that take place in Ischia.
Elena goes to Ischia during the summer of her fifteenth birthday, and the Sarratore family is staying in the same place as Elena. Elena has romantic feelings for Nino and often tries to speak to him at school and during this beach vacation. Nino will only stay in the beach house when his father, Donato, is not there.
In a scene with Elena and Nino in Ischia, Nino says to Elena, "I will devote my life . . . to try not to resemble him." He goes on to say about his father, Donato, "He would hurt anyone and never feel responsible." This is ironic because, later in the book, Nino leaves Ischia suddenly in the night without saying goodbye to Elena, and Elena is so upset she cries all night and tries to run to catch up with Nino at the ferry before he leaves. Nino hurts Elena but does not feel responsible; this happens throughout the series. This behavior is one parallel between Donato and Nino.
In other ways, Donato and Nino are very similar: they are both intellectuals, which is referenced by the novels that Nino is reading in Ischia and the way Nino discusses Dostoyevsky's The Brothers Karamazov throughout the Ischia scenes. Similarly, Donato is the first published author that Elena has ever heard of—his book of poetry, Attempts at Serenity, is discussed with reverence and as a sacred object for Elena.
Lastly, Elena idealizes both Donato and Nino for their similarity in not being like the men in the neighborhood. Elena discusses Donato as an ideal man, an ideal father and husband. She sees him as soft, poetic, and in touch with his feelings, unlike the other men she knows, who are prone to violence.
However, Nino and Donato differ in their affect: Nino is very serious and studious and doesn't talk much at first, and Donato is talkative, sings every evening, and is friendly and smiling. In the climax of this section of the book about the Sarratores, Donato sneaks into Elena's room in Ischia at night and touches her sexually without her consent. Elena feels confusion and anger about how she idealized this man who has raped her, and she leaves Ischia early the next morning.
Toward the end of the book, at Lila's wedding, Nino surprisingly attends the celebration, and Elena is worried that he will be recognized as Donato's son by the woman that Donato discarded ("I became anxious that Antonio's mother would recognize Nino"). The fact that he is not recognized allows Elena to continue her romantic crush on him, inferring that because Antonio's mother does not project her recognition and anger at Donato's behavior onto Nino, Elena can do the same.