New Mandy was a living toy that Billy built in The Grim Adventures of Billy & Mandy episode "Tickle Me Mandy".
She had created when Billy becomes lonely after Grim suggests he make a replacement Mandy. They make her from the items after Billy points out that she has no mouth until Billy points out that Mandy never smiles.
Billy activates it bringing her to life. She then asks Billy if he wants to play, which he happily obliges. Things are going great until she steals his very expensive crayons as she breaks all but one and uses it to draw all over his living room until she then throws a temper tantrum.
Billy is now frightened of her and doesn't want anything to do with her. Billy goes to bed and notices New Mandy in his room still asking to play. Billy screams and calls for Grim. Grim explains to him that she doesn't need to sleep, like he does, and that she will be waiting for him in the morning.
Billy has a nightmare that New Mandy terrorizes him and beats him with a spatula. The next day the doll is waiting for him on the seesaw. He reluctantly goes off to play with her when he notices the real Mandy arriving back, and is overjoyed. Mandy spots the doll and asks who is she, the doll replies "Hi I'm Mandy WANT TO PLAY?"Mandy explains that she is the only Mandy in this neighborhood and slams the other side of seesaw to the ground causing New Mandy to go flying into the sky, fall down to Earth and smash to pieces.
They hold a funeral for the doll and Billy is in tears. Mandy asks him why he is crying if he didn't even like it. Billy says that he wants to be alone, when suddenly her hand shoots out of the ground and grabs his nose shouting "play, play!".
It is unknown if she's actually alive or not as she does not make any more appearances or cameo in the show, although it could Implied Billy most likely crushes it.