Alphys is a yellow, reptilian monster hiding under a plain white bedsheet, since she is so shy. She wears glasses over the sheet, and her tail and feet can sometimes be seen sticking out of the bed sheet.
Alphys is really shy and self-conscious, so she covers herself in a sheet with eyeholes. Sometimes, when Alphys just wants to get away from everyone, she sneaks into the Ruins and lies on a pile of leaves. She is probably thinking about anime. She doesn't eat in front of anybody because she'd have to take her sheet off if she did.
She has a REALLY big crush on Undyne, but she would rather die than let her find out. If she is not watching anime at home, she is watching Undyne's show. When Undyne confesses her love to the mysterious girl covered in the sheet on her show, Alphys hides in the Ruins, only to be found by Undyne. The two have a friendly chat about anime, and Alphys shows her face to a stranger for the first time. The two are now the most well known couple in the Underground, thanks to Undyne "accidentally" mentioning it on her show.