On the third episode of the 2nd season, Cassidy introduces Tulip to Dennis whom he has known for a long time. Some have made the prediction that this is Si Coltrane from the comic books. If you have read the comic books, you know exactly what happens with Si.
We will have to see how this Dennis character plays out on the show but there is a good chance they are basing this character off of Si Coltrane who is a detective gone bad in the comic books. You can research more if you would like to know more about Si Coltrane.