1) Chemical 1) Overcomer 2) Subduer 3) Suppresser 4) Suppressor 5) Surmounter 6) Yoke 
• (n.) One who, or that which, restrains.
Re·strain'er noun One who, or that which, restrains. 
[n] - a chemical that is added to a photographic developer in order to retard development and reduce the amount of fog on a film 2. [n] - a person who directs and restrains
restrainer 1. A person who directs and restrains. 2. A chemical that is added to a photographic developer in order to retard development and reduce the amount of fog on a film.
noun a person who directs and restrains