The Slayer waiting for the Doctor outside the TARDIS
The Slayer is a clone of the Doctor created by the Daleks to predict his operations and movements, boosting their abilities to prevent him from foiling their plans. However, what they didn't comprehend was that the Slayer was too much like The Doctor and he ended up betraying the Daleks completely.
The Slayer then travelled round the universe destroying Daleks in their masses, and fulfilled his role as the 'Slayer', but this time as a Dalek Slayer. The Doctor remained almost completely oblivious to the Slayer's actions until their paths crossed once more, but this time they worked together as allies.
He is quite cunning...
His Dalek nature makes him quite ruthless when he wants to be, also quite cunning. The Doctor side of him makes him incredibly smart, and he is more strong than humans, has two hearts, but he cannot regenerate. The only way to replicate him to his former personality is to clone him again completely, making him lose his memories. Despite this, he can heal himself not unlike the Doctor when he regenerates, but he cannot heal himself from fatal injuries.
His personality is nothing like either of his two sides, however. He often shows extreme cases of cowardice, something both The Doctor and the Daleks find annoying. He also shows cases of bitchy two-facedness, However, he is loyal when he wants to be. He is awesome. He is The Slayer.
Because he is incredibly vain, he may also have a homosexual attraction to The Doctor, maybe because they look identical, and is always mentioning how "extremely handsome he is looking today!" His voice is exactly like The Doctor's, however The Doctor doesn't go all high-pitched when confronted by enemies. Nor does he pee his pants. Yes, The League is sorry to admit this happened once. Luckily, Dalek Dry-Cleaning saved him there.