Learn Hardcore Greek: Κανείς δεν είναι εδώ τώρα.

Greek uses what’s called negative concord: multiple negative elements work together to express one negation, not to cancel each other out.
In this sentence, κανείς is a negative/indefinite pronoun (“no one / nobody”), and δεν is the normal verb negator (“not”).
So Κανείς δεν είναι εδώ τώρα literally has “no one” + “not is”, but the meaning is still just “no one is here now”, not “everyone is here now”.
If you remove δεν, the sentence becomes ungrammatical; you can’t say ✱Κανείς είναι εδώ τώρα to mean “no one is here now”.
In Greek, when you use κανείς with a verb, you almost always need δεν (or another negative like μην) in the same clause for the “nobody” meaning in statements.

Can κανείς also mean “anyone”, or does it always mean “no one”?

κανείς can mean “no one / nobody” or “anyone”, depending on the context.

With δεν or other negatives, it means “no one”:
- Κανείς δεν είναι εδώ. → “No one is here.”
In questions, conditional clauses, or some other non-negative contexts, it usually means “anyone”:
- Είναι κανείς εδώ; → “Is anyone here?”
- Ξέρει κανείς; → “Does anyone know?”

So the same word switches between “no one” and “anyone” based on whether the clause is negative or not.