Joseph Crowe is an exceptional chemist who moonlighted as a guard for Arkham Asylum so that he could steal a vile of Joker Venom and vile of Fear Toxin. He then recreated the compounds and became Jo Crow.

Please critique

I also thought about throwing in Poison Ivy's pheromones, but are they a compound like the Joker Venom and the Fear Toxin, or is it one of her abilities?

I like that you're thinking about making your own characters.

My thought on this though is to maybe not make him so derivative. Some people criticize Batman's allies and enemies as being different versions of the same thing.

Joseph Crowe seeking out and using the compounds of existing villains make him a softer version of something that already exists.

An alternative take could be that Jo Crowe has been exposed to these compounds as a result of incidental contact with these villains like he was in a Bank the Joker attacked or a citizen who has residual effects of the fear toxin being loosed in the streets of Gotham. As such, he is now chemically unbalanced, etc.

I actually thought about doing something similar to that, where he was hit simultaneously with both Joker's Joker Venom and Scarecrow's Fear Toxin, having the resulting combination be where he is now able to control the emotions in others.