Mr. Sayoc has a protracted history of largely minor skirmishes with the authorities, but he now faces an especially grave array of charges in connection with the wave of attempted bombings, including illegal mailing of explosives and threats against a former president. Mr. Sayoc will eventually be prosecuted in Manhattan, and could be sentenced to a maximum of nearly 50 years in prison if convicted. He is not expected to enter a plea on Monday afternoon when he appears before Judge Edwin G. Torres in Federal District Court in Miami.

Sarah Baumgartel, a federal public defender in New York who was appointed to represent Mr. Sayoc, declined to comment on Sunday.

It is not clear how, or even whether, Mr. Sayoc will contest the charges against him, which the authorities brought after matching forensic evidence to him. Relatives like Ms. Villasana have begun to depict him as a man who has led a fractured life, beginning when Mr. Sayoc was a boy and his father left the family. He grew into an introverted teenager who liked to collect things, eat in his room and fall asleep with the television on, Mr. Sayoc’s family said, and they came to believe that he struggled with dyslexia. He clashed with his parents over his behavior, and Ms. Villasana said that her brother spent at least some time at a boarding school.

After he left college, apparently without completing a degree, the siblings’ maternal grandmother, who lived nearby in Hollywood, Fla., took Mr. Sayoc in against the wishes of his mother, who thought he was old enough to be on his own. By then, Mr. Sayoc was an enthusiastic bodybuilder, Ms. Villasana said; once she opened her grandmother’s refrigerator and found what she believed was Mr. Sayoc’s stash of steroids. She confronted him, telling him that he could get himself and their grandmother in trouble.

“I said, ‘What are you doing?’” she said. “He was always in the kitchen. Loved his protein shakes.”

Even so, Mr. Sayoc’s life was relatively stable while he was with his grandmother, Ms. Villasana said, but after he stopped residing there, his life became more nomadic, traveling the country working with male revue shows. His sister lost track of him periodically; until a reporter asked her about Mr. Sayoc’s 2004 divorce in Oklahoma, she did not know about his brief marriage.