|Affiliation||Chicago Police Department|
|Occupation||Civilian Tech and Surveillance (Intelligence Unit) (former)|
Expert Army Ranger (re-enlisted)
|Portrayed by||Samuel Hunt|
Greg "Mouse" Gerwitz, served alongside Intelligence Detective Jay Halstead with the Army Rangers. After a brief stint as a CI of Halstead's, Mouse joins the Intelligence unit as a civilian tech and surveillance expert, filling in the position left after Jin died. In Season 3, He is held hostage by Jeff Frazier in order to find the location of his daughter. In Season 4 he leaves the unit and returns to Army Rangers.