An orphaned thief with ever-shifting alliances -- Fish Mooney, Oswald Cobblepot, Bruce Wayne, herself? -- Selina Kyle has proved time and again that, above all else, she's a survivor. No longer the "helpless" street urchin saved from kidnappers by Jim Gordon, she's been an important ally not only to him but also to Bruce, whom she helps to save on more than one occasion.
When she threatens to expose Bruce Wayne's clone, Subject 514A, to Alfred Pennyworth in Season 3, Selina is pushed out a window and her body swarmed by stray cats in an homage to 1992's Batman Returns. She's found by police and taken to a hospital, where Ivy Pepper nurses her back to health with the help of her plants. Her relationship with Bruce strained, Selina turns to Tabitha Galavan, and tries her hand at the bullwhip, saying that she's "done just surviving"; she wants something more out of life.
In a city whose corrupting influence is virtually inescapable, Harvey Bullock has somehow climbed his way out of Gotham's gutters. Introduced in Season 1 as a crooked cop as likely to beat a suspect as he is to do the bidding of Fish Mooney, Bullock nevertheless becomes a loyal partner and valued mentor to Jim Gordon.
After being demoted, Bullock resigned from the Gotham City Police Department and found work as a bartender. However, he rejoined the force following an attack on the precinct for the group of ex-Arkham Asylum inmates known as The Maniax, and became acting captain after Nathaniel Barnes was hospitalized. He remains in that position at the end of Season 3, where he's reunited with Jim, now free of the Tetch virus.
Alfred Pennyworth & Lucius Fox
Bruce Wayne moved a giant step toward becoming Batman in the Season 3 finale, when he took to heart the words of a hospitalized Alfred Pennyworth, who said, "Find something you really care about, and you protect it at all costs, and when you find it, you'll never be lost again." Alfred was undoubtedly meaning a person, but Bruce's "true north" turns out to be Gotham itself, and in the final moments of the season, the teen donned a mask to begin his crusade against crime.
As Bruce sets off on that perilous path, we'll see Alfred moving from guardian and mentor to a more familiar role, as (occasionally reluctant) operational support, moral compass and master of excuses ("Rock-climbing!"). Likewise, tech genius Lucius Fox, the former Wayne Enterprises executive turned GCPD science expert, will quickly learn Bruce's secret, and become the young vigilante's armorer.