Larks was a sorcerer who briefly possessed the Murder Skull also known as Skulduggery's original Skull. He wasn't really present in the book series and was only briefly mentioned in Dark Days. His true name refers to a humble, ground-dwelling bird: suggesting that he may only have been a sorcerer of moderate power or ambition.
It is possible that Larks my have been a Necromancer: this would explain his purchase of a skull connected to so much death, as it would have helped bolster his power. However, this has not been verified.