Almost nothing is known about Dario other than that he wanted to be a novelist, but due to his mother's influence, he resigned from his dream and became a salesman instead. He and his family were apparently not local residents of Raccoon City, as Dario was registered at the Apple Inn, suggesting he was in town on business.
Raccoon City Incident and Death
"But this looks like the end for the great Dario Rosso, novelist extraordinaire. Cut down before his prime..." — Excerpt from Dario's memo about his failure to make his dream come true before his death
During the evacuation of Raccoon City on September 24 when the town was consumed by an epidemic that turned most of the population into zombies, Dario and his family were forced to fend for themselves, attempting to escape the encroaching infected hordes through the ruined streets of Raccoon City without protection or chance of aid.
Some time during the collapse of the city between September 26 and 28, Dario's mother, wife, and daughter were killed in the chaos. Being the lone survivor of his family, when Dario finally reached the safety of the warehouse, he was already a completely broken and scared man. Some time later, Dario was found by another survivor, S.T.A.R.S. officer Jill Valentine, that suggested risking leaving the site in order to reach the evacuation zone. However, Dario scolded her and refused to follow her, locking himself inside a shipping container in the delusion that it would be the safest option. While inside the container, Dario wrote a memo where he expressed his despair at believing that he would end up dying like his family and regretted that he failed to become a novelist.
Dario's final fate or how he died is uncertain because the accounts contradict each other. One of them indicates that, eventually, Dario left the container, but, as Jill had alerted, the warehouse was not a secure location; zombies managed to invade its interior and devour Dario when he tried to leave. If this holds true, Jill would have found his body when she returned at warehouse. In another account, after having locked himself inside the container, Dario preferred to stay there rather than trying to escape the city. In this case, he was never seen again, leaving what happened to him after Jill left the building unknown since she was unable to return to the place. Overall, in both accounts, Dario was last seen in the warehouse.