lone wolf

Short definition from Wiktionary

Noun

1. 1
   A wolf that is not part of a pack.
2. 2
   A person who avoids the company of others; a loner; an independent or solitary person.
3. 3
   A criminal who acts alone, not as part of a group.

Proper noun

1. 1
   A town in Kiowa County, Oklahoma, United States.