Roboute Guilliman

Roboute Guilliman is the Primarch of the Ultramarines, and the thirteenth son of the Emperor. Of all the loyalist primarchs, Guilliman is the only one whose current whereabouts in official canon are known.

Official Canon
Roboute Guilliman was found on Macragge, a stable world where, under the guidance of Guilliman's adoptive father, he created the realm of Ultramar, a vast empire located on the Eastern Fringe of the galaxy. When the Emperor found Guilliman, he was reunited with his legion, whom he named the Ultramarines, after his homeworld. He and his legion were lauded as model citizens of the Imperium of Man; even if they didn't have as many military victories as the likes of Horus, the worlds they brought into the Imperium were always stable and well-integrated to better serve mankind as a whole.

During the Great Crusade, the Emperor discovered that his son Lorgar had created a religion where he worshiped his father as a God. As punishment, the Emperor had the Ultramarines destroy one of his cities, and to have Lorgar and his legion bow down to the Emperor, Malcador, and Guilliman. Lorgar never forgave his father and brother for this humiliation, and turned to Chaos.

Horus Heresy
Guilliman felt guilty for the treatment of his brother, and so when the Ultramarines met with the Word Bearers on Calth, he did not anticipate when his brother would attack him and leave Calth a barren wasteland. While Guilliman wished to come to the aid of his father, massive Warp storms appeared around Ultramar that made such a voyage impossible. Instead, he activated a psychic beacon that guided his remaining brothers to Macragge, and announced that until they had proof that Terra had not fallen, he would create the Imperium Secundus. To ensure that it would not appear that he was willingly seceding from the Imperium, he and Lion El'Jonson elected Sanguinius as its ruler.

Eventually he and the other legions arrived on Terra, but they were too late to assist in the battle, The Emperor had been slain, and the remaining loyalists routed the traitors.

Post-Heresy
Guilliman wrote the Codex Astartes, a book that would guide each space marine on various tactics, strategies, and organizations. However, the key part was that it called for the breakup of the Space Marine legions into chapters numbering no more than 1,000 men. The surviving loyalists were split on the issue, and a civil war threatened to tear apart the Imperium again. It was only after Rogal Dorn relented that any conflict was avoided.

During the hunt for the Traitors, Guilliman managed to encounter Fulgrim, and the two dueled each other. However, Guilliman caught a poisoned blade to his neck, and was forced to retreat. Unable to treat the wound, his men had him put into stasis in the hopes that something could be done to revive him.

Gathering Storm
Note: The following information, while canon in WH40K, will be altered for inclusion in TTS.

Guilliman was finally revived from Stasis through the combined efforts of Tech Priest Belisarius Cawl and the Eldar herald Yvraine. Given a new set of power armor and the Emperor's own sword, Guilliman took back leadership of the Ultramarines from chapter master Marneus Calgar. Upon seeing the dismal state of the Imperium, he made a beeline for Terra, but not without opposition from Chaos, including Magnus the Red. However, he had the Grey Knights, as well as Cypher and his Fallen, by his side. The Imperial Fists, Adeptus Custodes and Sisters of Silence also came to his aid on Luna. Upon reaching Terra, he became suspicious of Cypher due to the sword on his back and had him arrested, only for Cypher to mysteriously escape. After speaking with the Emperor, he reorganized the High Lords of Terra, and set to work on fixing the Imperium, just as Chaos had succeeded in expanding the Eye of Terror and were on their way.

TTS Canon
One of the Emperor's first demands is to have Guilliman's life support shut off permanently. It would appear, given his overall annoyance with the Ultramarines and the Codex Astartes, that the Emperor wanted to get rid of him. However, the Emperor also said that he wanted to reunite his sons, and he wasn't about to let a little thing like death stop him. What the Emperor's plans are for Roboute Guilliman are unknown at this time.

Trivia

 * Roboute Guilliman is derisively referred to as the “Spiritual liege;” this is in reference to a comment made by a Games Workshop Employee named Matt Ward, who claimed that most Space Marine chapters look up to the Ultramarines Chapter Master Marneus Calgar as their spiritual leader (the character who was originally called the Spiritual Liege). The term is now used to mock Games Workshop’s perceived favoritism of the Ultramarines over other factions and their supposed superiority and perfection.
 * Ironically, Guilliman's role post-Gathering Storm is remarkably similar to that of the Emperor in the TTS-verse, working to help the Imperium be more in-line with his father's original ideals. Knowledge of the Inquisition and Ecclesiarchy enraged him and made sure that reforming them was at the top of his list of duties once returning to Terra. Guilliman even wrote a book containing the true history of the Imperium, free of propaganda and indoctrination, just as the Emperor suggested to Ecclesiarch Decius XXIII in Episode 18: Banished Expectations, called the Codex Imperialis.