General:
* Hermit: A person who lives in solitude, often for religious reasons.
* Recluse: A person who lives in seclusion or apart from society.
* Solitarian: A person who enjoys being alone.
* Ascetic: A person who practices severe self-discipline and abstains from all forms of indulgence, typically for religious reasons.
* Stoic: A person who accepts what happens without complaining or showing emotion.
More specific:
* Misanthrope: A person who dislikes humankind and avoids human society.
* Loner: A person who prefers to be alone.
* Introvert: A person who is predominantly concerned with their own thoughts and feelings rather than with external things.
* Eremitic: Relating to hermits or hermit life.
* Anchorite: A religious recluse who lives in solitude, especially one who lives in a cell or small dwelling.
Literary:
* Wistful: Having or showing a feeling of pensive longing or regret.
* Melancholy: A feeling of pensive sadness, typically with no obvious cause.
* World-weary: Tired of life and its problems.
* Despondent: In low spirits from loss of hope or courage.
* Alienated: Feeling isolated and estranged from other people.
The best word to use will depend on the specific context of your writing.