Perturabo

"'Dare you attack my NEWEST DAEMONIC FORTRESS OF PERPETUAL PAAAAIN?!?'"Perturabo is Primarch of the Iron Warriors, and fourth son of the Emperor. He also Smells like Dirt and Entitlement.

Official Canon
Perturabo was found on Olympia, where he had previously been a peace-loving reformist who sought to replace the corrupt government of his homeworld with a more enlightened system. However, by the time the Emperor found him, something changed his personality drastically. His first order upon gaining command of the Iron Warriors was to enforce the practice of decimation - not for any offense that they had committed, but to prove a point about how expendable they were.

Great Crusade
What had once been a proud workhorse legion, the Iron Warriors had been reduced to a meat grinder that spent the lives of its men like currency. This gave both the legion and Perturabo a grim reputation, with many of his brothers openly disgusted by him. However, this gave his legion a brutal efficiency, and the only men spared from this brutality were the most cunning strategists among his officers.

When Rogal Dorn claimed that he could build a fortress that not even Perturabo could destroy, he was beyond furious. The lack of respect he received from his brothers finally drove him over edge, and thus when Horus offered to turn traitor, he accepted without hesitation.

Post-Heresy
Having failed to destroy Dorn's defenses during the Siege of Terra, Perturabo swore he'd have his revenge; thus did he build a trap that was designed to humiliate Dorn and his Imperial Fists. By the end of the battle, the Imperial Fists were so effectively demoralized that they broke up as a legion, while the Iron Warrior took many trophies to sacrifice them to Chaos. Thus did Perturabo win the favor of the Chaos Gods, and became one of the only other Chaos Undivided Daemon Primarchs, besides Lorgar. He has no respect for the Chaos Gods, and only sees them as a means to an end.

TTS Canon
Perturabo has yet to emerge outside of flashbacks, but Rogal Dorn states that his intense jealousy is due to being a petulant manchild. The Emperor confirms that he frequently lashed out with tantrums whenever he didn't get his way. Perturabo is also mocked by Kitten and Magnus the Red for his noxious attitude and his reputation for "digging holes."

Personality
Described as a "petulant manchild", Perturabo's exact personality is heavily obscured by Rogal's own intense dislike for him. Within the flashbacks, he is often seen goading Rogal and poking fun at him, calling him degrading names such as "Rogass" and "Rouge Gurl". He has also asked rogal to fist him. It is unknown how accurate this is.