Optimus's death

Bravery and courage. Sideswipe is not afraid to engage in battle and is ready to fight to the end to achieve victory. In battle, he proves himself a skilled warrior and a master of hand-to-hand combat.
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Impulsive and reckless. He often acts without considering the consequences and enjoys extreme situations. He enjoys taking risks and doesn't particularly mind the prospect of getting hurt. This behavior often leads to trouble, which is why Sideswipe regularly ends up in Ratchet's care.
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Narcissism. His remarks often ooze self-satisfaction, such as the famous line, "I'm damn good!"
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A tendency toward manipulation. In some storylines, he uses cunning and deception to achieve his goals, suggesting less-than-honest methods.
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Attitude to rules and authority. Sideswipe often ignores orders and prefers to act independently, which creates tension in his relationships with his teammates. He's not particularly fond of discipline and can be rude to his superiors.
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Caring for his comrades. Despite his selfishness, in critical moments, Sideswipe is capable of brave and selfless acts to save his friends.
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Character evolution. The character's emphasis shifts across different media franchises (G1, films, Robots in Disguise). In some versions, he's initially portrayed as a rebellious teenager who learns teamwork through trials. In others, he's a more mature fighter for whom protecting Earth has become a new, higher purpose.
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Overall, Sideswipe is a complex character: he has youthful enthusiasm, courage, and serious flaws, but at the same time he is capable of real feats for the common good.

Magnus was named his successor and passed on the Matrix to him. However, Magnus was unable to open the Matrix and harness its power. He later became Chief of Staff under the new leader, Rodimus Prime.
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Other Notable Autobots
Bumblebee. One of the earliest and most popular heroes. On Cybertron, he served as a spy and reconnaissance agent. On Earth, he became a friend and vehicle for Spike Witwicky, the Autobots' first Earth ally. As a result of a serious injury, he lost the power of speech and communicates through sonic signals and fragments of songs. He later led a squad of Throttlebots—the Metroplex garrison. At the end of season three, during the "Hate Plague," he was mortally wounded but was reconstructed into a more powerful Golden Giant.
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Ratchet. An Autobot doctor and scientist. He's known for his compassion—he even heals his enemies. After the war, he remains on Earth. He transforms into an ambulance.
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Arcee. A warrior. She lost her partners and now guards a human, Jack Darby. Transforms into a blue motorcycle.
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Bulkhead. A strongman. A former construction worker, clumsy but kind. He's Miko's guardian. Transforms into a green SUV.
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Wheeljack. Demolitionist and katana master.
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Smokescreen. Newbie, passionate fighter.
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The Dinobots. A special squad of Autobots that transforms into mechanical dinosaurs. The iconic five are: Grimlock (commander, a Tyrannosaurus rex), Slug (a Triceratops), Snarl, Airages (Spitfire), and Fennec. They were created by Wheeljack and Ratchet based on discovered dinosaur skeletons.

Here's a biography of Crosshairs, an Autobot from the Transformers universe. I've compiled the essentials: his past on Cybertron and how he performed in the films.
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Crosshairs
Biography
On Cybertron, Crosshairs served in Optimus's science division. When the war broke out, he joined the Autobots. He oversaw the launch of the Nemesis and later helped deliver Sentinel Prime's invention to the temple in Simfur.
When Optimus Prime devised a plan to rescue the Allspark, Crosshairs voluntarily remained on Cybertron. In his absence, hostilities continued: Decepticon patrols combed the planet, and many Autobots were killed or captured. Crosshairs managed to survive and even assembled a small force.
Later, Arcee returned to Cybertron. Crosshairs and his comrade Clocker learned from her of Megatron's death, the destruction of the Allspark, and the emergence of Starscream as the new leader of the Decepticons. He told Arcee that Elita-1 had been captured and taken to Simfur, and announced that he was preparing a rescue mission.
When the Autobots reached the Simfur Temple, they found a large force of Decepticons gathered there, awaiting the activation of the Allspark recreated by Starscream. A frontal assault was impossible, so Crosshairs suggested splitting up: he sent half of the squad, led by Arcee, to Trypticon to create a diversion and distract the enemy, while he and the other half remained near the Temple. During the battle, when Dreadwing's drones, on their master's orders, attacked Starscream and the remaining Decepticons, the Autobots took advantage of the confusion. As a result, the second Allspark was destroyed, and the Autobots escaped before Dreadwing's ship destroyed Trypticon.

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In films
Transformers: Age of Extinction. Along with the other Autobots, he hid in the Texas desert, awaiting Optimus's return. He participated in the attack on the KSI building in Chicago. When Optimus was captured by Lockdown, Crosshairs infiltrated his ship and sabotaged it. Then, in

The Fallen is an ancient Transformer from the Transformers film universe who actually has ties to the Decepticons. I'd say he wasn't formally the faction's original leader, but in the story, he became its supreme leader and is considered the founder of the Decepticon movement.
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Who is he?
The Fallen was originally known as Megatronus Prime. He belonged to the line of Primes—the most ancient and powerful Transformers born of the All-Spark. Along with his brothers, he traveled the galaxy, searching for sources of Energon and exploring the universe.
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The Primes once imposed a ban: it was forbidden to destroy stars in systems harboring life. But the Fallen violated it. Arriving on Earth around 17,000 BCE, he assessed the Sun's energy resources and decided to destroy it, even though this would also destroy Earth. He placed a device (the "Star Extinguisher") on the site of the future Great Pyramid of Giza.
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The other Primes opposed the plan, and war broke out. The Fallen won the war, but was unable to activate the mechanism: the brothers hid the Matrix of Leadership (the key to the system) and locked it away. After this, the Fallen lost the right to be called a Prime, his name was forgotten, and he became known only as "the Fallen."
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Later, according to one version, he was imprisoned in a spatial "pocket" (for example, on Phobos, a moon of Mars), from where he was freed by Megatron. The Fallen became his mentor, and the Decepticons obeyed him.
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Role in the film "Transformers: Revenge of the Fallen"
It was Fallen who became the main antagonist of the 2009 sequel. He returned to complete what he had long ago begun: finding and activating the "Star Harvester" hidden on Earth. To do this, he needed the Matrix of Leadership.

Megatron is one of the central characters in the Transformers universe, the leader of the Decepticon faction and the main antagonist. He is the sworn enemy of Optimus Prime, the Autobot commander.
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All Hail Megatron Poster
Origin and Backstory
According to one version, Megatron began his career on Cybertron as a gladiator, naming himself after one of the Thirteen Primes, Megatronus. At that time, he was an ally and even a friend of the future Optimus Prime (then known as Orion Pax), the planet's archivist. However, over time, power corrupted Megatron, and he became obsessed with world domination.
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When Cybertron's resources were depleted, the Autobots, led by Optimus Prime, abandoned the planet in search of new energy. Megatron followed them on his cruiser, the Nemesis, but the ships were caught in a meteor shower and crashed on prehistoric Earth. The Decepticons remained dormant for four million years until Megatron awakened in 1984.
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Character and Motives
Megatron is a complex and multifaceted character. On the one hand, he is intelligent, insightful, and a talented strategist who knows how to exploit the weaknesses of both enemies and subordinates. He is demanding, does not tolerate objections, and harshly punishes disobedience. On the other hand, he is alien to mercy and compassion. His main goal is to establish his dominance first on Cybertron, and then throughout the universe, believing that Transformers are a superior race that should rule over all others. He views humans as an inferior life form and is willing to destroy humanity if necessary to achieve the Decepticons' goals.
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Powers and abilities
Physical Strength: Megatron is one of the largest and strongest Transformers. He stands 12.7 meters tall and weighs 5.7 tons. He can triple his weight at will to simply crush an opponent.
Weapon

Knock Out is a character from the animated series Transformers: Prime. He is a Decepticon who combines the roles of a medic and a ruthless fighter.
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Knockout Transformers artwork, Transformers art, Transformers cybertron
Appearance and alt-form
Knockout transforms into a red sports car resembling an Aston Martin DBS V12. He's extremely proud of his polish, and the slightest scratch sends him into a rage.
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Character and role
Knockout is a cunning, vain, and narcissistic Transformer. He admits to preferring "breaking things rather than fixing them," and is more drawn to combat than healing. In combat, he relies on speed and cunning, often using sneak attacks.
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His main partner was Breakdown, a powerful enforcer who embodied physical strength. Knockout often entrusted him with "dirty work" to avoid taking risks himself. After Breakdown's death, Knockout became enraged and thirsted for revenge.
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Abilities and weapons
As a medic and scientist, Knockout possesses valuable skills. He is armed with:
An energy shock spear that can stun enemies with electrical discharges.
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A circular saw that his left hand transforms into.
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With a heavy-duty drill from the right hand.
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These instruments were originally intended for medical purposes, but during the war they became torture devices. In vehicle mode, it is also equipped with twin blasters.
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Key plot points
Knockout performed experimental surgeries on Megatron, including transplanting the arm of one of the Primes.
He participated in street racing among humans and clashed with the Autobots, particularly Bumblebee.
After the capture of the Nemesis and the death of Megatron, Knockout voluntarily surrendered to the Autobots.