Swensen was a Starfleet security officer stationed on board the USS Enterprise during the 2260s.
In 2268, he and Bardoli were part of a landing party led by Captain Kirk that was searching for the dikironium cloud creature on Argus X. (TOS: "Obsession")
Later that year Swensen was called to the transporter room where he kept his phaser trained on Jojo Krako when the latter beamed aboard the Enterprise. (TOS: "A Piece of the Action")
Still the same year, he was among the security personnel who fought against Kang's Klingon crew near main engineering, under the influence of the Beta XII-A entity. (TOS: "Day of the Dove")
Background information Edit
Swensen was played by an unknown actor, who also appeared as an Organian villager in "Errand of Mercy" and a Babel Conference delegate in "Journey to Babel".
The same actor also appeared as a member of a security rescue team, in "Catspaw", however his appearance ended up as a deleted scene. 
In "Obsession", Kirk calls him "Henson" (and corresponding subtitles also say "Henson"), however the script calls him "Swenson".