The noun recluse means someone who lives alone and deliberately avoids other people.

Synonyms are hermit, solitary, ascetic, or monk.

The word origins from 12th century meaning “person shut up from the world for purposes of religious meditation” and from Old French reclus meaning “hermit, recluse” also “confinement, prison, convent, monastery” (source also from Late Latin reclusus, past participle of recludere meaning “to shut up, enclose”. It is derived out of its assimilated form of Latin re-, intensive prefix, plus claudere meaning “to shut”.

That billionaire is a recluse who refuses to give interviews.

You can never make her come to any social outing. She had always been a recluse at heart.

They found it amusing that their Psychology professor was a recluse.