Origin
Sniffer was an assassin who was hired to kill Daredevil (golden-age) using his remarkable strength, endurance and his phenomenal sense of smell, failing this he was sent to prison, he tried several other times to kill Daredevil with an equal lack of success.
Later while in prison he and eleven other former foes of Daredevil and Crime Buster were offered a chance at a full pardon if they worked for the Allies in the Pacific theater of World War II, accepting this they became known as the Deadly Dozen.
On his return from the War Sniffer reformed, more or less, and became very nearly a hero if you were generous with the term hero.
Creators
Starting as a grotesque villain to two different superheroes, Sniffer was later turned into a sort of a superhero himself and given a backup spot in the pages of Daredevil Comics.