He's the one person you can rely on in a situation like this.
Synonyms and examplesonlyShe's the only one who can do it.
singleYou haven't listened to a single thing I've said!
lonePolice were looking for a lone gunman.
soleHe was the sole survivor of the plane crash.
Go to the thesaurus article about these synonyms and antonyms ofone.
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If thereare no other people or things like this one, you say it is the only one.