You can unlock Steve in the Battle Game minigame by solving the drawer puzzle in Chris's game. There is a painting of a volcano on a small piece of land, which reads "Lava burns forest, flows to sea, forms the earth." The drawers must be selected in this order: Red, Green, Blue, Brown. This opens a nearby drawer containing the Luger Replica.
No weapon can kill him in CODE: Veronica. However, he can be stunned to stop him from hitting you with his axe which can lead to a dying state if hit (first aid sprays or combined herbs are recommended in the case of being hit by Steve), the magnum and grenade launcher are recommended to stun Steve, but be aware that he recovers very quick, after stunning him continue your way to escape from the room you just got in, after escaping from the room a cut-scene will play and Steve will die after saving Claire from Alexia.
Alternatively, just use first aid sprays and mixed herbs each time hit and keep running. Steve can only get in about two or three swings if the player continues to run to the cutscene trigger.
The Darkside Chronicles
Steve is playable during the Game of Oblivion scenario for chapters 1-6 but not chapter 7 which is the last chapter. His handgun skin is appears to be a luger pistol, based on his use of the Golden Lugers from CODE: Veronica. His counterattack is first kneeing the zombie in the gut then preforming drop kick and landing on his feet. His assist attack is first delivering a jumping kick then preforming another type of jumping kick involving turning his body to kick with force causing his body land on the ground and needing to get back up. His costumes are;
Prisoner - Default costume.
Western - Extra costume. Unlocked by getting an "S" rank on all Game of Oblivion chapters on NORMAL. Notably features the phrase "Axe Monster" on the back of his vest with axes printed under the words. This is referencing his use of axes when he becomes a monster.
Steve is fought while he is mutated by the T-Veronica virus during Game of Oblivion 7. Like his CODE: Veronica incarnation, no weapon can kill him in Darkside Chronicles. He can still be stunned to stop him from hitting you with his axe with either a fully upgraded handgun or a shotgun, provided the player shoot him in his exposed heart to both stop his attack and inflict extra damage (it is recommended to save the powerful weapons for the battle with Alexia). When you start climbing the destroyed stairwell (since you're fighting Steve in a larger area compared to his original CODE: Veronica fight), Steve will jump in an attempt to hit you once again. Simply use a fully upgraded handgun or a submachine gun to take him down. When you reach the highest point of the stairwell, Steve will make a last attempt to kill you, use the handgun to take him down, then press the command prompt to dodge his final attack in which he destroys the area where you were standing, after the fall you'll see that Steve drove the axe into his own neck, after removing it, he'll fall in a weakened status ending the fight. Following this, a cutscene will play in which, like in the original CODE: Veronica, Steve will save Claire and die.