When watching the Supergirl trailer you may have not recognized some of the characters, but that will be sorted out right now.
Melissa Benoist portrays Supergirl also known as Kara Zor-El/ Kara Danvers. A 24 year old Kryptonian who is the cousin of Superman. Kara lives in National City working as Cat Grant's assistant at CatCo.
Mehcad Brooks potrays James "Jimmy" Olsen, who was a photographer Daily Planet and who has actually been in contact with Superman. He now works for CatCo in National city. Olsen will also be the love interest of Kara.
Jeremy Jordan will be portraying Winslow Schott who will eventually become Toyman, one Superman's Villains. Schott works alongside Kara at CatCo and it seems in the trailer that he has a bit of a crush on her.
Then there is Hank Henshaw who becomes Cyborg Superman, which may not happen in the first season, it would be a Harrison Wells/ Barry Allen kind of situation for those of you who watch The Flash. He is an ex-CIA agent who now works at the Department of Extra-Normal Operations (DEO).
Chyler Leigh plays Kara's adoptive sister, she works for Henshaw as a doctor at DEO.
Finally we did see the episode 1 villain, who is Vartox played by Owain Yeoman. Vartox is an alien convict, who has been hiding on Earth for twelve years and seeks a battle with Supergirl after she emerges.