In nominative case, all nouns, including names, take an article in front of them. E.g:
You're hanging out with a friend. Suddenly, you see Eleni near by. You turn to your friend and say: "Η Ελένη!"
On the other hand, nouns aren't accompanied by articles in vocative case. E.g:
You see Eleni and want to grab her attention. You yell at her: "Ελένη!"