Gotham: Harvey Bullock
Harvey is a long-time, on-and-off-again, member of the Gotham City P.D. initially mentoring (and later, good friends with) young Jim Gordon. In a police force as corrupt as Gotham's, an officer doesn't last long if he's not willing to get his hands dirty. Whether Harvey's drinking problem sprang from those necessary evils or vice versa is really not the point. He's proven he can be better than he's been and serves as better police than that city really deserves.
Arrow: Moira Queen
The matriarch of the Queen clan was the picture of the high-class, highly-capable lady of status. The public saw a loving mother, resilient widow, and principled business woman so it was no wonder when she chose to run for mayor. What came to light was her involvement in a business cabal with nefarious goals, an extramarital affair with Merlyn which resulted in Thea, and her knowledge (and hiding) of Oliver's son. Like many on this list, she made these deals with the devil for well-meaning purpose but her last moments seemed infused with silent regret.
black-ish: Earl "Pops" Johnson
Pops is the epitome of "Do as I say" sort of mentoring. Andre and Ruby regularly point out the various vices (smoking, gambling, drinking) and morally ambiguous (con games) activities he revels in on a seemingly daily basis. However, to his grandkids, he does appear to be the one adult in their world truly living his best life and enjoying the heck out of it.
Brooklyn Nine-Nine: Captain Raymond Jacobs "Ray" Holt
A credit to the uniform, Holt is known throughout the NYPD as a principled officer of valor and professionalism. He has overcome racism and homophobia to earn his captainship and takes the time to understand his officers in the interests of making them better at their job, thus improving the 99th as well. He owns his past gambling addiction and uses that experience to empathize with those still struggling with their own personal challenges.
Rick and Morty: Rick Sanchez
Grandpa Rick is probably the least suitable and most effective mentor for anxious and overlooked Morty. Rick's constant reckless but brilliant schemes never fail to drive home some sort of lesson and, so far, Morty's survived them all. Well, SOME version of Morty has survived.
Into the Badlands: Sunny
Sunny is the killer with a conscience in the world of The Badlands. Like Gotham's Harvey, he's become what he needs to be to survive. Doesn't hurt that he's REALLY good at killing. He mentors M.K. so the boy can train to be a Clipper versus laboring as a Cog. M.K.'s trust in him is destroyed when he is finally able to remember that his mother and the camp they were living in was slaughtered by Quinn's Clippers, led by Sunny.
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