Ensign Rizzo was a male Human Starfleet officer who lived during the mid-23rd century. He served in the operations division aboard the USS Enterprise under Captain James T. Kirk where he was assigned as one of the ship's security officers.
Prior to their assignment together onboard the Enterprise, Rizzo 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 cloud. He barely survived the creature's attack and 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. (TOS: "Obsession")