Dalek Sec

Dalek Sec was the leader of the Cult of Skaro.

Origins
Dalek Sec was at first the Commander of a Dalek force whose mission was to wipe out the Mechanoids. Sec commanded from on the battlefield and overlooking the field from vantage points. After the Mechanoids were exterminated, Sec returned as ordered to the Emperor. There, 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 newly-housed in Metalert Dalekanium that was more defensive than the average Dalekanium. Sec, along with the rest of the Cult, then went on to begin using their initiative to defeat enemies. (DWF: Birth of a Legend) During the Last Great Time War they managed to escape the carnage by hiding between realities in a Void ship. (DW: Doomsday)

The Battle of Canary Wharf
Emerging from the Void Ship 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 unhindered by the Cybermen's invasion, stating "This is not war - this is pest control!" Sec led the Daleks to the top floor of Canary Wharf (Torchwood Tower) where the Cult of Skaro opened the roof and and ascended to an area 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 who were also 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. (DW: Doomsday)

1930s New York
Sec along with the Cult of Skaro ended up in New York City in the 1930s 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". (DW: 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 fueled 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 mere minute later he was the next of the Cult to die, at the hands of the human Daleks. Dalek Thay was killed shortly thereafter, leaving only Caan, who performed an emergency temporal shift and disappeared. Exactly what happened to the casing he left behind, or indeed his body and that of his killers is unknown. (DW: Evolution of the Daleks)

Personality
Until his transformation, Sec was like any other Dalek, albeit more intelligent as noted by the Doctor. However 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 Dalek's hadn't been able to feel for thousands of years. He told the Doctor that the deaths the Dalek's 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 devoted to the Daleks but also felt that if they remained "pure" it would result in the destruction of their race. 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 Dalek's 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 better person than he had been as a "pure" Dalek.