Sean Donovan was a son of Mike Donovan, that lived apart with his mother, Marjorie Donovan. Before the Visitors arrival, he used to meet his father from time to time, something that his mother didnn't like very much. For her, the relationship with Sean was sort of a competition between her and Mike, in which she believed herself always to be the losing side.
After the arrival of the aliens to Los Angeles, Sean, just like many other teenagers in the city, became very excited about the idea of meeting the Visitors. One of his friends managed to acquire a model of a Visitor's shuttle and action figures of the most famous aliens, including the one of Diana. The boy was dreaming to have as much, so he asked his father to buy him something as cool as his friends had.
When Mike was accused of commiting serious crimes against the Visitors, Sean was one of a few people that didn't want to believe that. He still believed his father to be a hero and a good person.
Some time later, the Visitors began a full-scale invasion of the planet and took many humans as prisoners. In reality, the aliens transported them to the Mothership, where they would be stored for further conversion into food for the Visitors. Sean was one of the many held there. Mike tried to save the boy, but he failed.
It was his grandmother, Eleanor Dupres, that, being sympathetic to Visitors promises of helping people, asked the Visitors representative Steven to see into the matter and make her grandson return home. Sean was returned, but not before he was converted by the Visitors. Later, the Visitors would use him to spy on the Resistance.
Mike Donovan lost contact with his son for quite some time before he later found Sean as a member of the Visitors Youth recruit program. By that time, the boy had been brainwashed significantly and wouldn't listen to Visitors protesters.