Primus TFP

"But to destroy Primus is to destroy Cybertron."

- Megatron about Unicron's plans in Predacons Rising.

Primus is the living creator-God of the Transformer race, whose body became the planet Cybertron. Primus is the twin brother of Unicron, his eternal foe, whom he battled for eons before going into stasis. Primus also created the Thirteen, the first Primes, to defeat his evil twin brother and cast Unicron into deep space.

History
Primus was created in the beginning of the very universe it self by the One. However, the scale had to be balanced, which resulted in the creation of Unicron the destroyer. The two brothers battled on a deserted cyber world for ages, but their endless stalemate forced Primus to create the Thirteen original primes giving them the ability to transform allowing for more possibilities in battle. Primus and the Thirteen than confronted Unicron and eventually by bringing the most potent spark of all the thirteenth prime close enough to the anti spark, they managed to finally defeat and cast out the chaos bringer. With the battle ended at last Primus merged with the core of the planet by shedding his physical body and transferring his energy into the Core. Leading to the activation of the Well of Allsparks, Primus' entire body than became one with the planet which would become known as Cybertron and his spark serves as its Core, which produced an unlimited supply of Energon necessary for the continued existence of the Cybertronian race. Primus created the "Transformers" and is like a God to them as he gives them lives to live. In the early days of Cybertronian race Primus was worshipped frequently as a god, with massive temples, churches and monuments erected in his name but as the race progressed and advanced, the belief in Primus began to slip and eventually most of Cybertronian society believed him to be a myth along with Unicron and the Thirteen.

When the Great War began, Megatron had infected the Core with Dark Energon the very life blood of Unicron himself. As Primus was suffering from the effects, Orion Pax bravely went to the Core to save Primus. For his efforts and bravery, Primus gave Orion the Matrix of Leadership, which reformed Orion into "Optimus Prime". This had the side effect of unambiguously confirming the God's existence to the Cybertronian people but in order to survive the core had to shut its systems down completely leaving cybertron cold and barren until many centuries later the autobots were able to restore primus spark with the reconstructed omega lock.

In the end of Predacons Rising, Optimus Prime flies to the core, Primus, to become one with him and destroy the anti spark of Unicron for ever. Primus is watching over the seemingly deceased Optimus, as it appeared when Optimus was shined on by the light of the core. This would seem to be the new transformation that Optimus mentioned as he entered the core. Optimus may return some day as his red spark can be seen flying out with the other sparks from the well.

In The Buzz on Windblade, Windblade mentions that Primus upgraded her form, placed her in stasis and ordered her to protect Earth from any threat.

Personality
Primus' personality is unknown, but is presumed to be direct contrast to Unicron's. It has been said by many that Primus is very wise and benevolent. Primus may or may not speak like his twin brother Unicron does, depending if Primus has a vocal sound. Primus gives life and is a god to his creations. Many Cybertronians, including Ratchet, talk about Primus and how he gave them life.

Primus never speaks on the show, but there are flashbacks of him. In Predacons Rising, Primus was seen as Optimus entered through the Core to become one with him.

Primus makes up the core of Cybertron and Unicron makes up the core of Earth. Primus and Unicron never had a brother to brother bond. Since they're opposites, they fought countless times. Primus is pure good and Unicron is pure evil.

Powers
Primus's power is equal to that of Unicron. It has been shown that by his will alone he was able to create three species and manifest pure Energon. It is said that he possesses, like Unicron, a form of pure energy and can shed his physical body which Optimus stated, he already has. He presumably possesses any powers over Energon and Cybertron that Unicron has over Dark Energon and Earth.

He possess the ability to directly transmit information to another individual's mind as long as Energon flows through them, mirroring Unicron's ability with Dark Energon.

When Optimus Prime entered the Core to support life on Cybertron, Optimus was merged with Primus. Light shined on him as he went deeper into the Core. This allowed Primus to give life again and millions of sparks burst from the Well. Like Unicron, Primus likely possessed the ability to revive anyone who was dead, as Optimus was able to briefly return to life to defeat Underbite and speak to Bumblebee and his team in Robots in Disguise, possibly due to Primus. Also Optimus's statement about "the universe" having plans for him in the future, may refer to Primus.

Trivia

 * Optimus Prime is the first Cybertronian in history to have ever met Primus, the second being Windblade. Soon after the Matrix of Leadership was given to Optimus, Primus remained "dead" for some time until Team Prime took over the Nemesis and used it's Omega Lock to restore Cybertron. Primus was fully restored after Optimus sacrificed himself to bring life again on their planet.
 * Primus appears in Transformers: Robots in Disguise.