Sigma

Sigma is the main character of Son of Doctor Who. He has been portrayed by Tenotch Zarate and Sam Nichols.

First incarnation
The first incarnation of Sigma briefly appears in the pilot played by Tenotch Zarate. This incarnation preferred to be called Ace, though The Rani calls him Sigma, indicating Sigma is his original name.

Ace appears outside a high school where he meets his companion, Vincent "Vinny" Smith, he offers Vinny some Bazooka bubble gum, after which he asks what his fortune on the wrapper is. Vinny says the fortune is "A new friend is about to enter your life." Ace then asks him who thinks it is.

Sometime later, he gets kidnapped by the dying Time Lord and his father's enemy, The Rani, who is trying to absorb his regeration energy, Ace complies, having already felt his upcoming regeneration, and sends it to her all at once, killing her. Vinny enters the room, finding Ace on the floor, who urges him to stay back. Vinny is helpless as Ace regenerates into his next incarnation...

Second incarnation
The second incarnation of Sigma returns to calling himself Sigma, his first line after regenerating being, "What idiot told you my name is Ace?" After regenerating, Sigma left Vinny at the school where he went back to his TARDIS, called "The Brown" after companion Peri Brown.