fellow vs singleton

fellow and singleton both are nouns.

Word	Noun	Adjective	Verb	Adverb
fellow	Yes	No	No	No
singleton	Yes	No	No	No

As nouns, singleton is a hypernym of fellow; that is, singleton is a word with a broader meaning than fellow:

fellow (noun)	singleton (noun)
a boy or man	the playing card that is the only card in a suit held in a bridge hand as initially dealt
a man who is the lover of a man or woman	a set containing a single member
a friend who is frequently in the company of another	a single object (as distinguished from a pair)
a person who is member of one's class or profession
an informal form of address for a man
a member of a learned society
one of a pair