WordNet sense 1 (lacking companions or companionship):

alone, lonely, solitary

WordNet sense 2 (without accompaniment or companions):

unaccompanied

WordNet sense 3 (not seeking or given to association; being or living without companions):

unsocial

WordNet sense 4 (being the only one; single and isolated from others):

lonesome, only, sole

WordNet sense 5 (existing alone or consisting of one entity or part or aspect or individual):

single