The Glad Dummy is optimistic version of the Mad Dummy, his Undertale counterpart. He is usually helping Frisk with assistance throughout his moments in the game.
The Glad Dummy helps Frisk for a portion of the game, but departs after a certain point when he goes to try and make his cousins Napstablook and Mettaton make up. He also appears in the final battle in the pacifist route for support along with the other characters. During a Genocide Route, Glad Dummy will fight the player and become Mad Dummy.
The Glad Dummy, similar to Flowey, spares you to begin with.