Gil Mason is a deputy police commisioner under the command of Commisioner Gordon. But it was revealed he secretly works for Two-Face. He's the main antagonist of the two-part second season premiere of Batman: The Animated Series, Shadow of the Bat.
Mason's history prior to being in the police force remains a mystery. Somehow, Two-Face got Mason into the police and made him become deputy police commisioner through manipulation behind the scenes.When Gil first came on the job, he was shown to have good abilities as a cop and Gordon began to trust him and even said he was a godsend. He arrested Rupert Thorne and busted other criminal rackets due to a mysterious informative who was Two-Face. Gil is also involved romantically with Barbara Gordon who returns the feelings as well.
Gil later framed Commisioner Gordon for having connections to Rupert Thorne. After that he took Jim's place, and supported the bid for Gordon's bail. At the rally, Mason delivered a speech but it was interrupted by gunmen. Barbar disguised herself as Batman for the rally fought the gunmen but they got away. Robin noticed Gil ducked before the guns appear. She managed to get a good look at one of the gunmen's faces. Barbara suspected Gil was behind framing his father when she saw the gunmen at his house while trying to tell Mason of her finding. After figuring out Mason couldn't be trusted, Barbara took up the mantle Batgirl to see what she can find. Robin and her tracked Gil to an abandoned subway station. They found him questioning a man named Matches Malones (Batman in disguise). Batgirl got herself caught and with the help of Batman got out. The subway station was later flooded and Batgirl went on her own.
Two-Face had Gordon break out of prison to make Jim Gordon look guilty and eventually murder him. This would make Mason take over the job of Police Commissioner permanently and help Two-Face get his power over the criminal gangs in Gotham.. After Batgirl saved her fatheron the Gotham pier overlooking the harbor, Batgirl had a final fight with Mason on a speedboat as he tried to escape. She grabbed onto the rope still tied the boat and pulled herself up onto the boat while it was moving at high speed across the water. Mason fried his gundamaging the fuel tanks and setting fire to them. He succeeded in unmasking her and expressed surprise that Barbara Gordon is Batgirl but sustained injuries during the fight that put him into a coma. It is unknown if he woke up from his coma but was later taken into custody.
- Gil apparently never woke up from his coma since he would reveal the identity of Batgirl the first thing he woke up.