Green Greta is a white woman in a greenish dress with her cleavage exposed, gloves, high heels, a pair of sunglasses, brown hair, and a tattoo on her right breast. She is basically a recolor of Meesterious Mia.
She can only be used in Serious Sam HD: The Second Encounter if the player has the Serious Eight DLC. If the player doesn't have it, she cannot be used. Since the Serious Eight DLC cannot currently be obtained, those who don't have it won't be able to play as her for a long time, if ever.