Kanemaru (金丸) has a psycho gun on his right arm and wears sunshades. He use to be an assasin and his alias was Black Dragon. He was a member of the Old Yorozuya but he left. Gintoki mentioned that Kanemaru was supposed to be straight man of the original Yorozuya.
Kanemaru's character is a spoof of Space Cobra, another anime series that aired in the 1980s, see Episode 138 for more details.