Dalek Sec

Dalek Sec was the leader of the Cult of Skaro, and later, the first Human-Dalek hybrid. Known for being given the capability to think beyond the normal scope of a conventional Dalek, he eventually resorted to unorthodox methods to keep the Dalek species alive.

The act that ultimately doomed him was tampering with the genetic template of his species, which was considered an affront to the existence of pure Daleks, and repulsed the other Daleks under his command when he chose to undergo an evolution into a new form. His personality vastly changed following his transformation, going from a ruthless strategist to a pacifist, and his subordinates responded accordingly by going from following his orders unquestioningly to rebelling against him.

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!" Sec led the Daleks to the top floor of Torchwood Tower, where the Cult of Skaro opened the roof 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. Motivated by emotions beyond anger and hate, 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.

However, the other Daleks came to reject Sec's theories as heresy, and eventually betrayed him. Dalek Caan took control and fuelled the other human bodies with pure Dalek DNA, in place of Sec's distinct mixture of the two. When the new Daleks in human shape tracked the Doctor down, they took Sec with them, putting him in chains. Taken before the Doctor, Dalek Thay accidentally exterminated Sec when he stood between Thay and the Doctor, either intending to save the Doctor by sacrificing himself, or expecting Dalek Thay to obey him and stop the mutiny.

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. 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. (TV: Evolution of the Daleks) 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. Although he didn't possess any more emotion than any other Dalek, he was very creative in what he said to the Cyber-Leader to the point where those who didn't know better might question otherwise; he described destroying the Cybermen as "pest control" and boasted that the Cybermen were superior in only one respect: "You are better at dying". (TV: Doomsday)

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. (TV: Evolution of the Daleks)

Despite the Cult of Skaro being, in The Doctor's words, "above and beyond the Emperor himself", Dalek Sec, along with the rest of the Cult of Skaro, still revered him; when Rose brought up that she became Bad Wolf and destroyed the Emperor, Sec would've killed her on the spot had the Doctor not arrived to grab his attention. Sec was also willing to sacrifice himself for the good of his race; his top priority upon exiting the Void Ship was protecting the Genesis Ark, which contained millions of Daleks, above all else, including his own survival, and was willing to sacrifice himself in the pursuit of evolution and adapting, as he believed the Daleks had to change in order to survive, their race down to just 4 Daleks by that point. (TV: Doomsday, Daleks in Manhattan, 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 the Dalek Sec.

His human emotions are caused from merging with Diagoras' (obviously) 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.