Ensign Aquino was a Starfleet engineering officer assigned to Deep Space 9 under the command of Commander Benjamin Sisko.
In 2369, Aquino did not report for duty and Jadzia Dax asked Chief O'Brien if he'd seen him recently.
While Chief O'Brien and Neela investigated a tritanium source in a deactivated power conduit on level twelve, section eight, they found the tritanium remains of O'Brien's EJ7 interlock as well as organic remains. Doctor Bashir later confirmed that the organic remains were from Ensign Aquino.
Later, it was discovered that he was killed by Neela while investigating a security breach on the runabout pads and then brought into the power conduit. (DS9: "In the Hands of the Prophets")