VA: Jon Hamm (ENG), Tetsu Inada (JAP)

Alias: The False Prime, The Betrayer

Species: Cybertronian

Age: Unknown

Gender: Male

Relatives: Thirteen Primes (brothers), Primus (creator)

Occupation: Leader of Cybertron (formerly), Herald of Unicron (currently)

Affiliations: Cybertron (formerly), Unicron, Malus 

Personality

Initially, Sentinel was presented as a humble, courageous, kind-hearted and well-loved leader who was the living manifestation of peace and prosperity.

However in reality, Sentinel is actually an evil, manipulative, self-serving, fraudulent, traitorous, tyrannical, arrogant, egotistical, deceitful, treasonous, black-hearted, and treacherous overlord who willingly sold out his own race to the Quintessons and later Unicron to claim power for himself. He also was aware of how much of a power-hungry monster he actually was, as he brags about wanting to be a king and have power over others, such as killing his fellow Primes in cold blood.

Sentinel had lacked any form of empathy towards all other Cybertronians. He was also quite cowardly, as he would hide behind his troops instead of fight someone head-on, even when he was fully capable of holding his own in a fight. Ultimately, Sentinel ends up outing himself as a despotic, arrogant sadist; who cares for no one but himself.

After becoming the Fallen and as Unicorn's herald, Sentinel is now more ruthless and sadistic in nature, as he now desired both Cybertron and Earth along with other planets in the universe devoured by Unicron.

Biography

One of the first ever Cybertronians created by Primus, Sentinel served as the chief advisor to the original 13 Primes. This position earned him respect from his fellow Cybertronians, but Sentinel became increasingly power-hungry and envious of the Primes and their glories. In a bid for more influence, he arranged a secret meeting with Quintesson Commanders, which led the Primes to seek Sentinel's intel.

However, this meeting was a trap set by the Quintessons, and in the ensuing conflict, Sentinel betrayed the Primes and played a role in their downfall. Stabbing Nexus in the back, beheading Megatronus Prime when the Quintessons dogpiled him, using Nexus' cannon to shoot Solus Prime into a rock, and kills Zeta Prime in the head. Before Sentinel could claim the Matrix of Leadership for himself, Primus rejected Sentinel and deemed him unworthy of it due to his power-hungry nature, causing the Matrix to vanish.

Following this, he claimed the transformation cog of Megatronus for himself, which augmented his power to the level of the Primes. Only Alpha Trion managed to survive his greed, while the High Guard, the personal army serving the Primes, reject him, his attempts to claim power, and went into hiding on the surface after witnessing Sentinel's betrayal as he slaughtered the Primes. They continued to fight in the shadows, working to sabotage Sentinel, as they came to believe that might makes right instead of unifying all of Cybertron.

Eventually, he would be killed by D-16, now taking on the name of Megatron after Megatronus Prime, who led a revolution to destroy what Sentinel stood for, but was stopped by Orion Pax, who became the last of the Primes: Optimus.

However, Sentinel's body was salvaged by the Quintessons at some point who delivered him to Unicron. Unicron rebuilt Sentinel into the Fallen, his right hand and a rogue Prime ready to slaughter Autobots and Decepticons alike in Unicron's name....

At some point he arrived on Earth after discovering Cybertron's downfall due to the civil war caused by his actions, and later was approached by Malus who asked him to become one of their members in exchange for helping Unicron achieve his goal.

Powers

Cybertronian Physiology: Much like other Autobots and Decepticons, Sentinel is a Cybertronian, a mechanical biological being who is capable of transforming into alternate modes such as vehicles and other electronic devices. Sentinel as both a Prime and as the Fallen, can transform into a Cybertronian jet.

Dark Energon Empowerment: Due to being rebuilt by Unicron, his blood flows through him, dubbed Dark Energon, which empowers him more where he is strong enough to match both Optimus Prime and Megatron in terms of power.