This is the 552nd most frequent Serbian word.
нико
"Нико" means "no one" or "nobody" in English.
Here, 'нико' is used as a pronoun in the nominative case meaning 'no one', indicating the absence of any individual participating in the action.
Нико није дошао вечерас.
No one came this evening.
In this sentence, 'никога' is the genitive form of 'нико', used after a negation to indicate absence of anyone the speaker could have seen.
Нисам видео никога у парку.
I didn't see anyone in the park.
'Нико' is again a pronoun in the nominative case functioning as the subject of the sentence and is used to state that there's no person who expressed complaints.