I want a noun (a thing not a person).
Consider an electric engine. The 'increaser' would be a knob which would increase the flow of electricity and make the engine give more power. A better way to describe that would be an 'increaser' that creates an electric field which causes the engine to work better (I don't know if that's possible). That effectively changes the environment so the target does more.
Here are a couple of words I considered but rejected:
augmenter: but that seems to imply something is added on as opposed to make the original item more; and
booster: closer to the idea I am trying to say, but it sounds temporary and its effect could be by way of something other than increasing the original item. For example a rocket booster is a different smaller rocket which boosts the main rocket, it is not causing the main rocket to be larger and more powerful.