Dalek Sec
Dalek Sec is the leader of the cult and the Dalek attack during the Battle of Canary Wharf against the Cybermen. He shows his leadership by communicating to the enemy, namely, Rose Tyler, Cybermen and The Doctor, and commanding the other three Daleks in which they "Obey". Because there were only four Daleks outside the void, Sec fought alongside the other Daleks against the enemy, and kept checking the status of the Genesis Ark. However, as soon as the Ark was 'primed' and Sec escaped the Torchwood tower with the Ark, he commanded the entire army of Daleks from the sky while keeping a close watch on the Ark. Because the Cyberman Army and the Void Ship containing the Cult, the Genesis Ark, and the Dalek Army within had travelled through the void, they had all picked up 'void stuff' which, when the Doctor reversed the flow of the radiation, sucked both armies, the Genesis Ark and the Void Ship back into the Void. Dalek Sec and the Cult were able to escape the slaughter by initiating an Emergency Temporal Shift.
Along with the rest of the Cult of Skaro, he ended up in New York in 1930 and established a base beneath the Empire State Building. Taking control of the Empire State construction and forcibly converting humans into "pig slaves" to serve as a labor force, the Cult of Skaro also worked on experiments designed to evolve Dalek-kind into a new form. The Cult recruited the power-hungry Mr Diagoras, who seemed to think like a Dalek the most. Sec turned against the concept of Dalek genetic purity, viewing survival as more important and seeing human DNA as holding useful attributes, as the humans thrive as a species whereas the Daleks had become extinct. Using himself and Mr Diagoras as the test for the Final Experiment, he was transformed into the first Hybrid Dalek.
While in his new body, he began to feel humanity for the first time in his existence, being motivated by emotions rather than the traditional Dalek drive to conquer. Motivated by these, Sec asked the Doctor to help him create more hybrids using human bodies, and started to express his feelings that their creator was wrong. He decided to have the new race live on, different from the Daleks, since if they continued to try to be superior, they would eventually die. However, the other Daleks came to reject Sec's theories and eventually betrayed him while Dalek Caan took control and fueled the other human bodies with pure Dalek DNA. When the Daleks tracked the Doctor down, they took Sec with them, putting him in chains and accidentally exterminating him when he saved the Doctor by sacrificing himself. The Doctor showed his respect for Sec by 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 keep looking for death and destruction... then death and destruction will find you." Dalek Jast had dismissed the thought at the time, but a mere minute later he was the next of the Cult to die, by the hands of the Human Daleks. Dalek Thay was killed shortly thereafter.
It is worth noting that, due to his hybridisation from Dalek to Human Dalek, Dalek Sec has been the only Dalek to repent the deaths he caused, and the only Dalek who willingly sacrificed himself to save the Doctor.
His character as a Dalek/Human hybrid is not entirely clear, as, although he eventually feels humanity, it is sometimes also implied that he still had plans to cause destruction. An example would be when Dalek Caan becomes 'controller' of the Human Daleks; Sec responds to this act of treachery with "That was to be my position!"
