They dress in clean, black suits and carry automatic weapons. They're only ever seen together, and would be identical if not for the colors of their masks. Mr. Grin wears a black mask with a big, white, shark-like grin, while the colors of Smiley's mask are inverted.
They were part of Joker's forces during the Assault on Arkham Asylum, where they avenged Goat Head Priest by shooting several Arkham Asylum security guards during their rampage. They stood among the survivors as Joker tortured his psychiatrist, Dr. Harleen Quinzel, before breaking out of the asylum.