Detective Nick Gage is called St. Nick by the other cops in Gotham, because he's the best Christmas present Commissioner Jim Gordon's ever gotten. In the short time he's been in Gotham, he has a shocking number of convictions and arrests, and he's usually right on the money. From an interview with Bryan Q. Miller.
Jim Gordon is very fond of Gage, and even suggests to his daughter Barbara Gordon that she go out with him.
Stephanie first met Nick when she rescued him from some rioting thugs as Batgirl. A bunch of armed men had shot down a civilian and some paramedics, and were holding Nick hostage. Stephanie came in to save the day, dispatching most of the gang. Steph noted that Gage was "pretty", then quickly reminded herself to focus on looking like a tough and forbidding Batgirl. Gage knocked out a thug pointing a gun at Steph and then proceeded to palm strike another. Unfortunately, Gage's palm strike made the last thug drop his rocket launcher, and it went off.
There was an explosion, but neither Gage or Steph were badly hurt, and Steph helped Gage up. After babbling about expecting to meet a Bat, but not expecting the Bat to almost kill him (oddly he seems to blame Steph for the mishap with the rocket launcher), he awkwardly thanked Stephanie.
He radioed Gordon to tell him that "a Bat helped out", then asks which Bat she is. Stephanie scoffed at this and left, while Gage yelled, "What? I'm new in town!"
Later, Nick showed up a little too late to bag the Scarecrow, Stephanie having already defeated him. He yelled, "Freeze! Hands in the air!", but Stephanie said, "Not today" and threw a batarang into a pipe, releasing a lot of steam. When the steam cleared, she had vanished. Stephanie was polite enough to leave a note in red paint (or lipstick?) on Gage's car window, saying "Thanks for not shooting me. Love, Santa's Little Helper."
Nick and the Commisioner went out investigate the activities of Livewire and the relationship between the two really mirrors the one between Steph and Babs, right down to Jim cutting in and saying witty lines before Nick has the chance due to "old habits"
When Livewire attacks the two cops, Stephanie came to the rescue, cheerily waving at Nick and shouting, "Hey, you!" Gordon acted incredulous at this, and Gage protested, "Nobody bats an eye when you talk to the Batman!". Gordon responded, "I'm pretty sure Batman is legal, Detective."
Nick was investigating the same murder Steph was in Batgirl (third series) (9). While he's in the building, the Calculator called and infectsed both the worker and the security guard with some tech based virus that brainwashed them to follow his orders to kill. The security guard threatened Gage but Steph arrives in time to save him Gage greeted Steph "hey, you". She responded "Hey, you?" and he said "you know it's what you said when...never mind" and he noticed that the guard was running toward the window and not stopping just as Steph did. The guard threw him out the window, and with a grumbled "this is gonna 'suh-uck..." Steph roped him with her batarang to break his fall. However, holding up a guy three times her size puts a strain on Steph. Even as Gage came to help her, the Calculator remotely terminated the man's life. Then, the Batrope slipped and the man fell/
Steph tries to throw herself after him, but Gage restrained her. She yelled at Gage to let her go, but he said "not on your life." She punched the wall in her anger and grief. They take the man to Ghoul the coroner, who performed an autopsy and sees the man has the same weird metal in his blood as previous victims.
After that night, Nick Gage stopped by Babs' office. When he saw she was out, he decides to do some impromptu investigating and found a newspaper clipping about Batgirl. Step walked in, saying she didn't know breaking and entering was his thing. She imagined flirty ways for him to respond, but he replied that that's an oddly personal statement from someone he doesn't know. She realized he hadn't guessed her secret identity and flopped down, asking him what brings him to Professor Gordon's office. He told her that Babs is really the only person he knows and that he "lost" someone last night, which hadn't happened to him in a while. Steph blurted out loud that this was really awkward accidentally, and realized she has inner monologue issues. She tried to tell Gage that there are people looking out for him, and he told her she was being weirdly personal again. She said "I am who I am", and they say goodbye and he left. Steph thought to herself "Way to sell it, Dork Knight. Way to sell it."
Nick meets up with Steph again in Batgirl (third series) (13) to take down Clayface which began with him down in the Financial District, where he attempts to coax a man out of hiding who is shooting up the block. The man retaliates by throwing a can of gasoline, causing an explosion, then sets off running on foot, Gage goes after him but the man immediately becomes Clayface and shoves him off the side of a building, right as Batgirl swoops in to save him.
Later, when Batgirl is having trouble fighting Clayface due to him taking on an identital form of her, Nick shows up and must decide which to shoot. He chooses correctly after the real Batgirl's clever prompt ("Shoot me...I'll bleed."), disarming Clayface.
Later, after Clayface has been defeated Gage tells Steph what Clayface’s motives were. A man named Basil Karlo, before he was Clayface, had a photograph of his late wife in Mrs. Griswald’s safe deposit box, and he needed her clearance to get in and see his wife again, one last time. Steph comments on the absurdity, obviously wounding Gage (“Romantic, but supremely messed up.” “Not as messed up as you think, Batgirl.”) as he walks off down the street, leaving Steph confused and concerned. It is apparent that Nick has lost a loved one in his life.
In Batgirl (third series) (15) the morning after a suspicious death, Detective Gage is examining his records, trying to shake out the mystery surrounding Newton (a geeky, science major who was murdered by cloaked figures) A police officer suggests suicide is the answer to how Newton died, but Gage knows better. Barbara Gordon decides to give Gage a bit of a hint by letting him know that there were “guys in robes” involved. Gage decides to investigate further just as his crew finds a blood-covered batarang. Hmm.