Learn Finnish: Tuo nainen, joka on puhelias, ei ole ilkeä. - That woman who is talkative is not mean.

Joka is the relative pronoun meaning "who", "which", or "that". Here, it is in the basic nominative form because it acts as the subject of the relative clause joka on puhelias (who is talkative). It refers back to the singular noun nainen (woman).

Could I use kuka instead of joka for "who"?

No. Kuka is a question word used to ask "Who?". When you are connecting sentences or adding describing clauses, you must use the relative pronoun joka.

Why are there commas before and after joka on puhelias?

In Finnish, you must use commas to separate a relative clause (a phrase starting with joka) from the main sentence. English often leaves these commas out, but Finnish strictly requires them to box in the extra information.

Why does the sentence use tuo instead of se for "that"?

Both can mean "that", but tuo physically points to someone or something you can actually see over there. Se is usually more abstract or refers to something previously mentioned in conversation. Here, the speaker is likely looking at the woman.

Why is the negative part ei ole instead of ei on?