Serious Sammy is a human woman with red hair (brown in HD) wearing a white crop top, a pair of jeans, and red sneakers. On the back of her shirt is a logo that looks similar to Serious Sam's. In Serious Sam HD and Serious Sam 3: BFE, she also has a dragon tattoo near her neck, a tramp stamp, and a pair of sunglasses.
Serious Sammy is Serious Sam's sister. She is quite strong, especially considering her size. This enabled her to be just as good as her brother, but unfortunately for her, she never got the recognition that Sam did.
In single-player mode's canon, Sammy is assuredly dead; even if she survived the ambush that killed Hellfire, Earth was destroyed right after Sam entered the Time-Lock, and Sam was the only person to get through. In co-op mode, who knows?