If someone you had never met in your life start saying

"no one liked you; no one wanted you, etc." behind your back.

Or someone you barely know or spoken to only once slandered you behind your back.

What do think of the character of such person?

Such saying clearly indicated malicious connotation.

Action speaks louder than word.

I don't pre-judged people.  I let people tell me who they are by their action, not what people said about the person.