aloof
Meaning: reserved, distant
Usage: The scientist could sometimes seem aloof, as if he didn’t care about his friends or family, but really he was just thinking about quantum mechanics.
insular
Meaning: separated and narrow-minded; tight-knit, closed off
Usage: Because of the sensitive nature of their jobs, those who work for the CIA must remain insular and generally only spend time with each other.
reclusive
Meaning: solitary, shunning society
Usage: Reclusive authors such as J.D. Salinger do not relish media attention and sometimes even enjoy holing up in remote cabins in the woods.
taciturn
Meaning: not inclined to talk
Usage: Though Jane never seems to stop talking, her brother is quite taciturn.
reticent
Meaning: reserved or reluctant to speak
Usage: The reticent student listened carefully but rarely joined the discussion.