Do not describe his general demeanor.
I believe in The Mentalist, Patrick Jane's origin was dissheveled, unshaven, suicidally depressed and he came wanting to help capture Red John, the serial killer that, because Patrick Jane mocked Red John on TV, killed his wife and young daughter, and left them dead in the bedroom for him to find when he got home.
He was a mess. Even so, through his unique skills (cold reading), he helped solve a case right away.
Don't dilute the impact of the depression, embrace it. This is what motivates him to work, he cannot stand to not work. Show him watching TV detective shows or something, bored and nailing the outcomes, but depressed doing it. Still it is the closest he can get to working.
Your longer (series) backstory is "Why is he this way? Why is Jack all work and no play?"
When he gets a job, he is transformed, and professional. Then the audience sees this side of him, but they should keep that original "mystery" depressed guy in mind, that is what lies beneath that professional. The depression gives him depth of character; don't dilute it by saying "this isn't the real Jack." It is a side of the real Jack, it happened, and it hurt him. Hopefully you will let us know how and why someday.