Ensign Rizzo was a 23rd century Starfleet security officer aboard the USS Enterprise under Captain James T. Kirk. He attended Starfleet Academy together with his friend, Ensign Garrovick.
When the Enterprise encountered the dikironium cloud creature on Argus X, Rizzo was part of a landing party assaulted by the vampire cloud. He barely survived the creature's attack, but lived long enough to be transported aboard and treated in sickbay. Rizzo, after an injection of cordrazine, was able to speak with his captain, long enough to support Kirk's contention that the cloud was intelligent. Rizzo died soon after the interview; a result of the creature draining at least 60% of his red blood cells.