- For his partner who uses the same signature balaclava, see Slade Wilson. For the comic book issue of the same name, see Wintergreen (issue).
- "Slade! Come back to die?"
- —Billy Wintergreen[src]
Bill "Billy" Wintergreen was a member of the Australian Secret Intelligence Service/ASIS unit, and the nemesis of Yao Fei Gulong. He was also the former friend and ex-partner of Slade Wilson, and the godfather of Joe Wilson. After he and his partner arrived on Lian Yu to rescue Fei, Billy betrayed Slade and allied himself with Edward Fyers' mercenary unit. Thus tracking and hunting down the man he was originally ordered to rescue.
Agent of ASIS
Billy and Slade Wilson were best friends prior to Lian Yu. They were so close to the point Slade made Billy the godfather to his son.
Some time before 2007, Billy and his partner and also friend, Slade, came to Lian Yu to rescue Yao Fei Gulong. Their plane was shot down, and Billy eventually allied himself with Edward Fyers, leaving Slade to escape by himself.
Working for Fyers
When Oliver was stranded on Lian Yu, he was captured by Edward Fyers' men who were looking for Yao Fei Gulong, their reasons unspecified. When Oliver refused to give up Yao's location, he was tortured by Bill. Oliver is later rescued by Yao, who engages Bill in battle and temporarily subdues him, escaping with Oliver.
Bill later tracked down Yao, besting him in a fight and taking him to a camp led by Edward. He was later seen fighting a prisoner in an organized fighting circle with all the other mercenaries watching. Billy throws many punches and kicks at the prisoner while the latter fails at every attempt to even touch him. Bill glances at Fyers who gives him a nod, allowing Bill to kill him.
When Oliver returned to the camp to rescue Yao, Yao knocked him out as Edward Fyers and Billy entered his tent. Bill was led to execute Oliver. However, Slade caused an explosion to distract everyone. Billy met Slade with vigor, eager to kill his old partner. As Oliver ran to his safety, Slade and Billy fought, leading to Slade stabbing Billy through his right eye, killing him.
Following the destruction of the Amazo, Slade was stranded back on Lian Yu and went searching for Oliver to enact his revenge on the latter. He stumbled on Fyer's former camp and discovered Wintergreen's corpse where he left it with the knife still stuck in his eye. After his return to Australia and at ASIS headquarters at Perth, Slade - not wanting to reveal the actual details about what happened on the island - lied about Wintergreen's betrayal and death to the regional head of ASIS, Wade DeForge, who was Billy's half-brother. Later on ASIS agents went to Lian Yu and discovered Billy's corpse, thus exposing Slade's lies.
Billy was once a man of honor who fought for his country, but after he and Slade were captured by Edward Fyers, Billy immediately accepted Edward's offer to work for him. It was after this moment, that Billy revealed his treacherous and self-centred nature. It can also be assumed that he had a bad temper, as when Oliver called him by his name and criticized him for what he had become, Billy brutally attacked him.
- Peak of human physical condition: Billy was a man in top physical condition and demonstrated incredible feats of strength, durability, speed, and agility. His reflexes were also fast enough to caught an arrow fired at him by Yao Fei, a superb archer.
- Master hand-to-hand combatant/Martial artist: Much like Slade Wilson, Billy was a master in hand-to-hand combat. His skill was enough to be on par with Yao Fei, who was also a skilled combatant, and eventually subdue him in their second confrontation. In addition to close range fighting, Billy is also well-versed in using acrobatics and high kicks in combat, which somehow contrasted Slade's fighting style of a solid close-ranged fighting. However, he was soundly beaten and killed by Slade, proving himself to be a superior fighter to Billy.
- Master swordsman: Billy used a sword as his primary weapon, and was extremely proficient when using it. His skill with a sword is at least on par with that of Slade's.
- Master marksman/Firearms: As a former ASIS operative, Billy was an expert in the use of various firearms.
- Protective suit: Billy used a variant of ASIS unit armor, as well as a distinctive orange-and-black balaclava, which was the same as his partner, Slade's.
- Curved Sword: Like Slade, Billy used a curved grey sword as his primary weapon.
- Guns: Billy used several different guns, including a pistol and machine gun.
- Knives: Billy also utilized many types of knives, including a machete.
- Grenades: Billy additionally used multiple types of grenades, many of which were strapped to his armor.
- "Damaged" (flashback)
- "Year's End" (flashback)
- "Burned" (flashback)
- "Vertigo" (flashback)
- "Betrayal" (mentioned)
- "The Odyssey" (flashback)
- "Dodger" (flashback)
- "The Promise" (mentioned)
- Arrow - Vengeance (corpse)
- Billy Wintergreen gave Oliver Queen his second scar by cutting the lower left side of his abdomen with a knife whilst torturing and interrogating him.
- Billy being stabbed in the eye foreshadowed how Oliver did exactly the same thing to Slade about a year later.
Behind the scenes
- In DC Comics, William Randolph Wintergreen is Slade Wilson's close friend and butler. He, too, was ultimately slain by Slade (despite being controlled by his son, Jericho).
- His costume and the fact that his eye is stabbed is a reference to Slade Wilson as Deathstroke. Billy was also referred to as "Deathstroke" in the credits of Arrow episodes.
- Billy Wintergreen never appeared without his balaclava during the show. His face was only shown in the tie-in comic books.
- A DC Collectibles action figure of Billy Wintergreen was released under the title "Deathstroke", even though he was never referred by this name on screen. However, the actor was credited as "Deathstroke" in the credits of multiple Arrow episodes.