Mouse's personality was mostly shown as always being jittery as well as having PTSD.
When he was interviewed by Platt, this was evident when she asked why he was on medical leave and he was unable to answer her so she stopped him and understood giving him the job.
Mouse is very close to Jay Halstead, as they served together and became close friends, and Jay was the one who helped him get the job in Intelligence. Jay was very protective over Mouse, which was seen when Mouse told him he was reinstating. Jay refused to let him go and tried to stop him, telling him that they both know what it does to a person and he can't risk going to the dark place they were both once in. However, he ended up letting him because Mouse insisted that he couldn't stand not being active on the field and even being a cop wouldn't fix that. So he left in Regarding This Wedding.
Chicago Police Department
U.S. Army Rangers
Left the army when his entire unit apart from Halstead where killed. Later re-joined stating he didn't know how to do anything else other than be in the Army.