Prima Prime: The Judge
The oldest of the Thirteen, Prima led the charge in battle and settled disputes among his siblings, often with a smile and a gentle tease. During the War with Unicron, Prima became increasingly withdrawn emotionally, to the point that some of the others would whisper to each other that they no longer recognized their oldest.
Once Unicron had been dealt with, the Thirteen began to settle and develop Cybertron as Primus went dormant within the core. However, the surface of Cybertron was still overrun with some of Unicron's twisted "creations". For this, as well as the burgeoning lifeforms that Onyx had taken to guiding and protecting, Prima saw fit to form various outposts across Cybertron.
(These outposts would go on to become the famous cities of Cybertron, the greatest of them later becoming known as Iacon.)
Eventually, a degree of peace came to the planet, and upon hearing from Onyx that the new lifeforms possessed rationality in addition to transformative abilities, Prima called Alpha Trion and Vector Prime to his side. Together they formed a unified law-giving body, though Prima took on the majority of this burden as Alpha Trion became occupied with maintaining the Covenant of Primus and Vector Sigma, the latter of which was made by Vector and Solus Prime to store vast Cybertronian knowledge, power, and secrets.
Between his duties as defender and judiciary, Prima requested a new device from Solus. He wished for it to be powerful, a symbol of protection, resilient against any attack, but with certain failsafes should he not have the Star Saber at hand. With this in mind, Solus created the Shield, capable of withstanding any punishment, creating a large, protective energy "bubble", and absorbing all forms of kinetic energy that could be unleashed from the Shield when all other forms of defense failed.
Despite this impressive resume, Liege Maximo found no end of fun in teasing Prima's "Skyboom" Shield with how any forceful impact connecting with the Shield resulted in a deafening/shockwave blast that could topple anyone who was not the user. Although Prima resisted the moniker, his second artifact forever became known as the Skyboom Shield once Solus and he, in rapid succession, slipped and called it such.
Liege made a point of keeping the original name of the Skyboom Shield from becoming public knowledge as he wove tales amongst the young populace and organized treaties between the cities. Although Alpha Trion recorded the truth in the Covenant with his Quill, he chose to omit that detail in his recordkeeping in the hopes of bringing Prima out of his shell.
War broke out when Onyx's strongest protege of a bestial branch gave in to rage against a supposed slight from Megatronus. Prima himself went to deal with the uprising, resulting in a legend of the First Predacon and his encounter with Prima that has been told (and occasionally exaggerated) over the millennia.
Following the First Predacon's defeat, Prima, against the wishes of Onyx and wisdom of Vector, exiled all the "beasts" to the uninhabited, but lush, Mithric Sea. Little did he know that such a beautiful, strong location would eventually decay into ravaged terrain and become a name associated with vice and sickness...
This decision temporarily split the three brothers, and Prima isolated himself between his duties from the others in his personal sparring chambers. It was during this time that the reclusive Sibyl Prime, the Seer, came to him with her Matrix of Leadership and spoke with him. What was said between them, even Vector knows little of. The results, however, were undeniable.
Prima returned to society, but with Sibyl at his side. Together, they formed through speech and action an interconnected Cybertron that aided and uplifted each city as needed. With Solus' aid, Sibyl managed to attune the Matrix to the Star Saber, and so she and Prima, affection blooming into romance, took the reins of planetwide leadership as their artifacts symbiotically shared power between the two Primes.
Shortly after Sibyl had gone to see Prima, Liege Maximo had withdrawn from the Thirteen. His absence began to concern them when, after a few centuries, he had not returned. Sibyl believed she knew where he had gone, and led them to the Great Sea. There, they found Liege Maximo imparting Prime knowledge to twelve "beasts" with greater organic appearances than the Predacons, and possessing higher intelligence than them.
Though Solus misinterpreted this as treason from Liege, Prima forestalled any attacks and asked for an explanation from Liege, as he was disobeying Prima's decree. Liege, in a surprising bout of passion, explained that these twelve - whom he called the "Covenant" - were the offspring of the Predacons Prima had banished. The reason they were not in the Mithric Sea with their kin was because they had been viewed as weak by their elders and thus abandoned long before the First Predacon challenged Prima. Liege had taken pity on them and sought to help them, a sentiment he would hold even if Prima banished him as a result.
Moved, Prima forgave Liege for his secrecy and allowed the "Covenant" to return to society in order to further their learning. Sibyl suggested that each of the Thirteen take one of the Covenant under their wing to, in time, pass on their particular specialties. This was agreed upon, and only Sibyl, seemingly, had no one to pass on her knowledge to - though she did have a servo in guiding each of her siblings' proteges.
It was through this that the modern Cybertronian population came to be, and all their Energon bloodlines can be traced back to the Thirteen and the Covenant. While most of the Thirteen formed sibling or parental bonds with a member of the Covenant and allowed their protege to receive a transfusion of their CNA, others - such as Vector Prime and Scorpia, as well as Liege Maximo and Virgol - formed more romantic connections with the Covenant member they had accepted.
Although he lived a while longer, Prima was the first of the Thirteen to die. Rogue Predacons had left the Mithric Sea and aligned with a few of Quintus' renegade creations, and Prima left the Skyboom Shield with Sibyl and took only the Star Saber with him to confront the rogues. The battle ended with him as victor, but it cost him his life.
As such, Prima lived to rest in the knowledge that he had performed his duties to the best of his ability, and died knowing that there was peace on Cybertron in spite of his shortcomings over the years...