Baron Helmet Zemo is a major antagonist who debuted in season 3 of Avengers Assemble (entitled Ultron Revolution).
He was voiced by David Kaye, whom also voices his alter-ego, Citizen V.
He is the enemy of Captain America and the son of Hydra scientist and commander, Baron Heinrich Zemo, who had previously fought Captain America in World War II. He is also the former leader of his own supervillain organization, the Masters of Evil, now known as the heroes who turned against him, The Thunderbolts.
As the son of Hydra's leading scientists and commanders, Helmet Zemo sought to carry his father's legacy and recreate the super-soldier serum. Zemo attempted to formulate his own version of the serum, but it was imperfect, leading his test subjects to turn into hulking abominations.
Now in his elderly years, Baron Zemo is resolved to find his father's perfected serum, as he brainwashes Captain America to lead him to an old Hydra castle, where he relives the moment of fighting his father there, alongside the aid of Bucky Barnes. Zemo soon makes his appearance before Iron-Man and Black Widow after they discovered Captain America and introduces himself to them. He then injects two of his Hydra soldiers beside him with his imperfect serum, mutating them before going into battle with the Avengers, while Zemo continues to follow Captain America into the castle. Zemo is finally lead to his father's laboratory, where he finds his father's serum and injects himself with it, rejuvenating him. Once Iron-Man and Black Widow find Zemo after defeating his mutated Hydra agents, they discover him in his new state as he now dons a mask. Zemo then battles the two and overcomes them until Captain America joins the fight after he breaks free of his mental control.
As they battle outside of the castle by a cliffside, Zemo fires his pistols at the castle's walls, causing it and the cliffside to crumble. Baron Zemo goes after the super soldier serum after it fell from him and towards the cliff, but Captain America grabs his hand as he tries to save him. However, Zemo refuses his help and instead falls into the water below, as he wishes to maintain his father's legacy. After the heroes get to safety, Iron-Man scans the cliffside, showing that Zemo had only disappeared.
Baron Zemo later recruits the Masters of Evil after he teleported them to his base from their previous defeat with the Avengers and appointed himself as leader. He then creates a diversion to lure the Avengers to his base located in Brasil, while he leads the Masters of Evil to Avengers Tower. However, once they take over the tower's systems and begin their raid, they discover Hawkeye as the only member there and assaults him. They finally capture him after chasing him throughout the tower. As Screaming Mimi subdues Hawking, she offers him a chance to join her team, but Hawkeye fools her by throwing her into a cell and escapes. Once Zemo acquired a device from Tony Stark's vault, Fixer alerts him of the Avenger's return, forcing the Masters of Evil to place the building on lock down from them.
The Masters of Evil plan to retreat after they noticing the Avengers forcing their way in, but Zemo advises them to stay and fight them on. Once the Avengers make their way inside, the Masters of Evil barricade each member and unleashes their training robots upon them while they deal with Thor themselves. Meanwhile, Baron Zemo fights Hawkeye single-handedly, who is disarmed with his bow and arrows. Luckily, Hawkeye finds another set of bow and arrows and fires an arrow at Zemo, forcing Zemo to flee. Though the Avengers corner him once they defeated the Masters of Evil, Zemo vows vengeance on them, before teleporting from the tower. While taking the Masters of Evil into custody, the Avengers discover that the only thing Baron Zemo stole from Tony's vault was an inversion stabilizer, which Tony states the device is designed to change facial and voice recognition.
Baron Zemo soon reappears as Citizen V, as he and the Thunderbolts (the Masters of Evil in disguise) arrive to help the Avengers defeat Growing-Man. Suspicious about their sudden appearance and popularity, Iron-Man and Falcon head to a warehouse facility, where it is the Thunderbolt's base of operations. As they confront with the Thunderbolts, Iron-Man reveals to them that they have stolen Stark Industries Technology, though the Thunderbolts deny this as they claim that someone else has stolen the technology. Suddenly, a high-tech explosive is dropped inside the facility, disrupting the hero's fight over each other that projects a holographic image of Justin Hammer, who the Thunderbolts reveal him as the thief behind Stark Industries. As Justin Hammer reveals to the Thunderbolts that he has caught wind of them knowing abouts his business, he sets his bomb to detonate and vaporize them. After the Thunderbolts and the two Avengers worked together to stop the explosive, Citizen V proposes that they team up to stop Justin Hammer, as he mentions him as their common enemy. However, Iron-Man tells him that the Avengers will handle Hammer themselves, before he and Falcon take their leave.
While the Avengers head out on their mission to various bunkers owned by Hammer, the Thunderbolts appear before them. Nevertheless, both teams side together as they fight on mandroids unleashed from the bunkers. The Thunderbolts and the Avengers then move inside and discover that Hammer is mass producing Mandroids there. Before either team could destroy the factories themselves, however, Hammer's hologram reappears and locks down the facilities, before they are set to explode with the heroes inside. After the heroes managed to escape, Citizen V reveals the Thunderbolt's publicist, Gabby Talbott, has been spilling information to Justin Hammer, in which enables the heroes to track Hammer's location.
Once they arrive at Hammer's bunker, they prepare for Hammer, whom suddenly appears piloting a giant mandroid him. While fighting him, Iron-Man pointed out that the mandroid is created from his technology and is shielded by a force field that is powered by a generator on its back side. The heroes disable the force field and destroy the giant mandroid, apprehending Justin Hammer, though Iron-Man still wonders how he obtained his technology.
At the Thunderbolt's headquarters, Citizen V contacts Gabby Talbott and tells her that she is fired. He then thanks the Avengers before they leave, and after the Avengers now assures their trust in the Thunderbolts, Citizen V claims that trust is exactly what they want, before he and his comrades head back inside their base.
The Thunderbolts later assist Hawkeye and Captain America with their battle against Ulysses Klaue in his Vibranium heist. Klaue overpowers the heroes with his sonic cannon and makes his escapes, but is then approached by Citizen V, who reveals his true identity as Baron Zemo to him. Once Klaue recognizes him, he realizes that Zemo was the one who arranged the heist and had set him up. However, Zemo informs Klaue that he is merely "building him up," as he knocks him out with his glider and places a small device on his belt. He then leaves Klaue to the Avenger's custody.
While Klaue is unconscious and incarcerated in the Avenger's holding cell, a device planted on the crate of Vibranium (similar to the one planted on Klaue) detonates, merging Klaue's body with Vibranium. As Hulk pummels him, Klaue increases in size and unleashes himself upon New York City. The Thunderbolts soon arrive and Atlas holds Klaue while Meteorite flies into his body and seemingly destroys him with Citizen V's sword.
Later, the Thunderbolts are watching a news report of how they outshined the Avengers in which Citizen V commends his teammates for their plan to pose as heroes, as he uses the stabilizer to reveal his true identity as Baron Zemo and the Thunderbolts as the Masters of Evil. Songbird, however, grew fond of the act of heroism and recalls to her teammates of their good deeds. However, Zemo demands her to follow his orders; otherwise he will use the scramble to reveal her true identity. The Masters of Evil soon detect Hawkeye in their base, who learned of their secret the moment before the Avengers then appeared. Once they break into battle, Hawkeye is fighting Screaming Mimi, whom he reminds her of what he heard from her as Songbird. This convinces her to turn against Zemo along with the rest of the Masters of Evil and join the Avengers as the Thunderbolts. As Zemo is by himself now, he collects Klaue's particles into his sword and summons a Vibranium device to emit powerful soundwaves that paralyzes the Avengers and the Thunderbolts. He then escapes the warehouse on his glider.
Zemo disguises himself as Citizen V at Times Square, where he is giving out a announcement about how the Avengers and Thunderbolts are now gone. However, he is then shocked to see the Avengers and the Thunderbolts alive before everyone. Captain America then appears before Citizen V and uses the stabilizer to reveal his true identity as Baron Zemo. Nonetheless, Baron Zemo fights the heroes on by using his sword to transform himself into a giant energized version of Klaue.