
Someone
Expectation of Familiarity
Implies the speaker expects you to know or guess who is being referred to.

You'll never guess what someone told me!
Privacy Respect
Used to omit identity when it should be kept private.

Someone special gave me this gift, but I can't say who.
Unknown Presence
Indicates the presence of a person whose identity is unknown to the speaker.

I heard someone sneaking around outside.
Compare with
AnotherAnyoneEveryoneFolkIndividualPersonWhoever
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