Deathstroke (Arrow)

Deathstroke is a villain from the Arrow Universe and the main antagonist of Season 2 completely.

Being one of the many Slades in the multiuniverse, he is the first one apart from the first Slade to take a major villain status. He was once a friend of main character Oliver Queen until the love of his life died and he blamed Oliver for her death and swore revenge on him. Sadly it was a misunderstanding

TGTTA 2
Deathstroke appears as an ally to Issac/The Author who was recruited when the Slade of Slade's Ensemble refused to join Issac.

Deathstroke is the right hand man to The Author or at least one of them. But he is mostly defiently his most trusted ally.