Dalek Sec

Dalek Sec was regarded by the Tenth Doctor as the most intelligent of the Dalek race. (TV: Evolution of the Daleks) He had the ability to think beyond the capacity of any other Dalek and eventually became the leader of the Cult of Skaro. Sec believed that the Dalek race needed to evolve by combining their DNA with that of other alien races in order to survive and planned a Final Experiment, which culminated in his assimilation of the human, Mr Diagoras.

Dalek Sec's assimilation of a human allowed him to experience human emotions which, along with his vast wisdom, resulted in his transformation from a cold, merciless strategist to a merciful pacifist. Due to this change in personality, his fellow Daleks unfortunately went from obeying him without question, to actively rebelling against him for going against their race's racial purity doctrines.

Origins
Dalek Sec was the commander of the Seventh Incursion Squad whose mission was to wipe out the Mechonoids. Sec commanded from on the battlefield overseeing the field from vantage points. After the Mechonoids were exterminated, Sec returned as ordered to the Emperor. Sec was selected as the leader of the Cult and given his name. He was then sent to the weapons research area for upgrades, where he was re-encased in Metalert, a type of Dalekanium that was defensively superior to standard Dalekanium. Sec, along with the rest of the Cult, began using their initiative to defeat enemies. (PROSE: Birth of a Legend)

During the Last Great Time War, they managed to escape the carnage by hiding between realities in a Void ship. (TV: Doomsday)

The Battle of Canary Wharf
Emerging from the Void Ship along with the Genesis Ark, a Time Lord device, Sec served as Dalek commander during the Battle of Canary Wharf against the Cybermen. He and the other Daleks seemed unfazed by the Cybermen's invasion, stating, "This is not war — this is pest control!" The Cult of Skaro opened the roof of Torchwood Tower and ascended to a space where the Genesis Ark could open.

Sec and the rest of the Cult of Skaro released an army of Daleks from the Genesis Ark, which started to fight the humans and the Cybermen present at the battle and in the streets below. The Tenth Doctor was able to open the void which caused the Daleks, the Cybermen and everything else which had travelled through the void to be pulled into it. Instead, Sec and the Cult initiated an emergency temporal shift and vanished before they were sucked into the void. (TV: Doomsday)

1930s New York
Sec, along with the Cult of Skaro, ended up in New York City in 1930 and established a base beneath the Empire State Building. The Cult of Skaro attempted to create new Dalek mutants and revive the Dalek race; when those attempts failed, Sec decided to attempt the "Final Experiment": blending Dalek with human. He enveloped Mr Diagoras and began to "evolve". (TV: Daleks in Manhattan)

Human hybrid
While in his new body, he began to feel humanity for the first time in his existence. At first, he felt Diagoras' ambition and his instincts for violence; he even commented that the humans' genius for war made them very much like the Daleks. Later his views changed. Motivated by emotions beyond anger and hatred, and rejecting the traditional Dalek drive to conquer, Sec asked the Doctor to help him create more hybrids using mindless human bodies. Sec started to express his feelings that their creator had been wrong. He decided to create the new race of Dalek humans, different from the Daleks, out of the belief that if they continued to try to be superior, they would eventually die.

The other Daleks came to view Sec's beliefs as heresy, however, and were repulsed by his decision to combine his DNA with that of a human as they no longer saw him as the pure-blooded Dalek that they'd previously served. Sec was betrayed and captured by his former servants, and ultimately sacrificed his life to save the Doctor from being exterminated by Dalek Thay.

The Doctor openly declared his great respect for Sec, calling him the "cleverest Dalek ever" and the only creature that could have led the Daleks from the darkness.

Dalek Sec made a prediction mere seconds before his death, stating, "My Daleks... understand this. If you choose death and destruction... then death and destruction will choose you." Dalek Jast dismissed the thought at the time, but a minute later he was the next of the Cult to die, at the hands of the Dalek-human hybrids, who refused to obey the Daleks' commands as a result of being infused with Time lord DNA, due to the Doctor's actions. Dalek Thay was killed shortly thereafter, leaving only Caan, who performed an emergency temporal shift and disappeared. (TV: Evolution of the Daleks)

Personality
As leader of the Cult of Skaro, Dalek Sec was more intelligent and creative than most Daleks; indeed the Doctor thought that he was the cleverest Dalek to have ever existed. Nevertheless, before his transformation, Sec was just as vicious as any other Dalek, mercilessly shooting down anyone who got in his way during the Battle of Canary Wharf.

He also showed a sense of sass uncharacteristic for a Dalek - when conversing with a Cyber-Leader, he claimed that wiping out the Cybermen across Earth could be done "with one Dalek" and that "pest control" would be a more appropriate term for such a slaughter than "war". Later in the conversation, he appeared to compliment the Cybermen by saying they were superior to the Daleks in one respect, but then said that the former were only "better at dying." (TV: Doomsday)

Although he displayed no mercy to his enemies, Sec was willing to sacrifice himself for the good of his race and believed that he was doing so by going through the "final experiment".

After using Mr Diagoras to transform into a human Dalek, Sec's personality changed and he began to feel compassion and mercy, feelings that the Daleks had been incapable of for thousands of years. He told the Doctor that the deaths the Daleks had caused were wrong and that their creator was also wrong to make the Daleks the way they were. He also displayed respect for humanity and commented that, despite their faults, they possessed courage.

Sec was very dedicated to the Daleks but also felt that "purity" had brought their race to near-extinction. He believed that the Daleks had to change and adapt to survive. He hadn't intended for his transformation to result in him gaining human emotions, but believed that these emotions were good and that if the Daleks accepted them, they would survive. Unfortunately his Dalek companions disagreed with him and this eventually resulted in them betraying him.

Sec tried to reason with the other Daleks either by demanding that they obey him due to him being their leader or begging them to listen to him. Sec eventually sacrificed himself to save the Doctor, showing that he had become a much better person than he had been as a "pure" Dalek. The Doctor showed outrage over his murder, referring to Sec as "the only creature who might've led [the Dalek race] out of the darkness". (TV: Evolution of the Daleks)

Other information
Dalek Sec, as with all Daleks that originated from the Last Great Time War, possessed markings below his eyepiece to identify him. (TV: Doomsday)

The Visual Dictionary
The non-narrative source Doctor Who: The Visual Dictionary gave further information about Dalek Sec.

His human emotions are caused from merging with Diagoras' human brain. His six tentacles (three either side of his face) are from merging. He gained dexterous fingers, and due to more-exposed-than-human flesh, this makes him weaker to attack.

After fully merging and leaving the Dalek, its casing, along with the weapons, were gotten rid of as they no longer served any purpose.

Dalek Sec