Kosinski aboard the Enterprise
Kosinski was an arrogant Starfleet propulsion specialist. His warp drive efficiency upgrade experiments brought him to the USS Enterprise-D in 2364. Despite the seemingly nonsensical nature of his warp equations, he had previously had success with two other Starfleet starships, the USS Ajax and USS Fearless. It later was revealed that it was his mysterious assistant, The Traveler, who was affecting the upgrades and not him. (TNG: "Where No One Has Gone Before")
Some of Kosinski's theories were still valid, and Ensign Wesley Crusher tested his warp equations aboard the Enterprise while it was docked at Starbase 133 in 2367. Wesley inadvertently created a warp bubble which trapped his mother, Doctor Beverly Crusher, in an alternate universe. In this universe, Wesley contacted Kosinski for assistance when crewmembers began disappearing, but he was unable to explain the phenomenon. (TNG: "Remember Me")
Background
Kosinski, played by Stanley Kamel in "Where No One Has Gone Before", appeared in a Starfleet uniform with no communicator and did not appear to have a Starfleet rank, as he is referred to throughout the episode as "Mr. Kosinski". Kosinksi's collar featured a unique rank insignia, a small silver rectangle and a darker rectangle of the same size. It could be that he is a civilian temporarily working for Starfleet and given a uniformed position (traditionally called a "warrant officer"). His condescending attitude towards Picard and Riker would support the argument that he was not a part of the service or its chain of command. He is also called "sir" by Riker on several occasions lending further credence to the theory that he is outside of the regular chain of command.