Kyle Nimbus, nicknamed the Mist by Cisco Ramon, was a former hitman of the Darbinyan crime family. He was affected by the S.T.A.R. Labs particle accelerator the night he was supposed to be executed via gas chamber and gained the meta-human ability to transform himself into poisonous gas. Kyle...
But like with the female Dr. Light, the faces and names of the former-dopplegangers change in the adjusted post-Crisis timeline, due to their importance in it.
That's my take on it.
Also, Photoshop edit I made.
The description for Alex Morf's version seems like the same as Anthony Carrigan's in Gotham, but I'll wait and see if he is a merged/altered version of him potentially.