Omega

Omega is an evil Time Lord. He was a great intergalactic engineer and co-founder, but his mind was twisted. After a long time in the Anti-Matter world, his body was being destroyed, with only his consciousness remaining.

Personality
Omega is very hateful towards the Time Lords, who believed that he sacrificed himself to attain greatness. He also developed mood swings and a vicious temper. However, he is enchanted by a child's smile and a steam organ, which suggests that his temper is simply a result of loneliness and despair from his stay in the Anti-Matter world.

Omega does have an extremely strong will, allowing him to create servants that suit him in Season 9. His physical form was created by his will alone.

Role
Omega is first seen in the Seventh episode of Season 2. He plans to transmit a signal from the planet Rapperius which will control the minds of everyone in the universe and make them rap like the Daleks. However, the Doctors foil his plan.

In the very next episode, he and Rassilon lock the Doctor's group in a museum filled with Vashta Nerada, intending for them to get eaten. However, they escape.

In the Season 2 finale, Omega leads an army of Slitheen to conquer London after claiming a Slitheen egg and cloning it. The Eleventh Doctor fights the Slitheen using a Space Marine Dreadnaught whilst the Tenth Doctor, Sclar, Skaldak and Ood Sigma wield a giant Super Soaker filled with vinegar to destroy the Slitheen. After the battle, Omega escapes.

In Season 3, he is a servant of Rassilon, and helps him to destroy the Medieval village with a Cyberking. In the Season 4 finale, he helps the Ninth Doctor and Morbius to try and conquer the Earth. After Morbius' death, Omega tries to get the Ninth Doctor to help him, but to his despair, the Ninth Doctor decided to join the winning side (The Eleventh Doctor's group) as UNIT leads Omega away.

In Season 9, Omega escapes from his imprisonment with help from his new servants, the Gelguards. However, in the season finale, he winds up getting captured again and imprisoned on a spaceship.

Omega isn't seen again until Season 13, where Azaxyr and the Cyber Controller release him from the prison ship and help him get revenge on the Doctors. In the eighth episode of the season, he gets a new suit of armor.