Parem
Parem was a Romulan officer accidentally phased by an interphase generator being tested aboard his ship in 2368.
When Lieutenant Commander Geordi La Forge and Ensign Ro Laren were also accidentally phased by the same device, he swiftly became aware of their presence aboard the Romulan ship. He followed them aboard the Enterprise, and threatened Laren while the two stood, invisible to all around them, on the Federation vessel's bridge. Forcing her at gunpoint to lead him to La Forge, who Parem believed could return them back to normal phase, Laren eventually resisted, kicking the Romulan.
The ensign led Parem on a chase through several bulkheads through the occupied quarters of unaware personnel. La Forge, following Lieutenant Commander Data as he tracked the duo's chroniton wake, joined Laren in the fight and kicked Parem through the outer bulkhead of the Enterprise, sending him careening into space. The two were later able to use Parem's disruptor as a signal flare to their comrades, making Data aware of their plight and enabling them to be rescued. (TNG: "The Next Phase")