Ratcatcher

Otis Flannegan was one of Gotham's most successful ratcatchers before he committed himself to a life of crime as the Ratcatcher. He was born with a strange affinity to communicate with, train, and ultimately control rats; unfortunately, he eventually decided to use these low-level superpowers to live a life of crime, drifting in and out of prison, and making a home in Gotham's sewers.

Biography
While not appearing in Joker, the Ratcatcher is possibly referenced by the news statement that giant super rats had suddenly started to pop up in Gotham City, which makes a short cameo appearance at Crime Alley near the end of the movie.