An introvert or loner was someone who preferred to keep to themselves rather than socialize.
In 2372, after correctly guessing that Tobin Dax was the former Dax host who tried sleight-of-hand, Julian Bashir reasoned his introverted nature made him naturally gravitate to such things. (DS9: "Rejoined")
Olly described herself as a loner. (LD: "Of Gods and Angles")