Ensign Rizzo was a 23rd Century Starfleet security officer aboard the USS Enterprise under Captain James T. Kirk.
When the Enterprise encountered the dikironium cloud creature on Argus X, Rizzo was part of an away team down to the planet. Due to the hesitation of his friend Garrovick , he was killed by the creature as 60% of his red blood cells were drained. (TOS: "Obsession")
- Rizzo was played by Jerry Ayres who also portrayed O'Herlihy in Arena.