Nephthys was one of the planned characters for Underfist: Against the Astrovampires and in the Underfist series. She is the Egyptian Goddess of Mourning and the daughter of Anubis, the Egyptian God of the Dead.
Nephthys is portrayed as a stern and relentless hunter who was sent out by her father Anubis to hunt down mummies who escape from their tombs to go on rampages and bring them back to their tombs. She tracks down and attempts to capture Irwin with the help of her adviser Winky (her snake friend on her helmet). Of course, Nephthys is also a little girl, so she likes ponies and makeup as well as hunting the undead. Also, according to one of Maxwell Atom’s Tumblr posts, she would have replaced Mandy as Irwin’s love interest.
Due to Maxwell Atoms contract with Cartoon Network expiring and and plans for the series have been cancelled, Nephthys never came into fruition, but she makes a cameo appearance during the end credits of Underfist: Halloween Bash in the scene where she rides on her scarab beetle jetpack chasing Irwin on his Fistbike.