When it comes to characters, it’s difficult to find one—more all-American than Supergirl. As played by fresh-faced Littleton native Melissa Benoist, the superhero and her alter-ego Kara Danvers have helped the DC Comics universe flourish on television — alongside fellow CW network shows “Arrow,” “The Flash” and “DC’s Legends of Tomorrow” — even as dour—big-screen counterparts—like “Batman V.