This was rough. I don't watch many TV shows so it was really difficult to find enough characters to match twelve gods and goddesses. This is my attempt at it.
Aphrodite - Goddess of love and beauty.
{aka Caroline Forbes} from The Vampire Diaries
Chosen because... well, she's my girl crush. That basically sums it up.
Apollo - God of sun and song.
{aka Kurt Hummel} from Glee
Chosen because he's an amazing singer and he belongs in the spotlight.
Ares - God of murder and bloodshed.
{aka Sylar} from Heroes
Chosen because in the few seasons I watched this he had a lot of blood on his hands.
Artemis - Goddess of animals.
{aka Brittany Pierce} from Glee
Chosen because she talks about her cat a lot and seems to enjoy animals and their company.
Athena - Goddess of strategy.
{aka Katherine Pierce} from The Vampire Diaries
Chosen because she's always got some sort of plan or scheme going on.
Demeter - Goddess of farming.
{aka Penny} from The Big Bang Theory
Chosen because she's from a farm town in Nebraska.
Dionysus - God of parties and alcohol.
{aka Damon Salvatore} from The Vampire Diaries
Chosen because he's always drinking and enjoys a good time.
Hephaestus - God of hard work.
{aka Artie Abrams} from Glee
Chosen because he always has to work twice as hard to keep up with the glee club.
Hera - Goddess of women and childbirth.
{aka Quinn Fabray} from Glee
Chosen because she had Beth, and because she's one of the lead girls in the show.
Hermes - God of thieves and travellers.
{aka Klaus Mikaelson} from The Vampire Diaries
Chosen because he talks about seeing the world a lot, because he has lived such a long life.
Poseidon - God of the sea.
{aka Sam Evans} from Glee
Chosen because he's a synchronized swimmer for McKinley High.
Zeus - King of the Gods.
{aka Peter Petrelli} from Heroes
Chosen because he can use any power he is exposed to, thus being the most powerful.