Who is Devil-boy Jack from The Haunting of Alaizabel Cray and what is their importance?

Devil-boy Jack is young and mysterious, with long, scraggly hair and eyes that have been sewn shut. He is a valued advisor to Lord Crott. Devil-boy Jack knows many complex spells and can foretell the future with scrying stones. He helps in the fight against the Fraternity, even though he knows that he will die in the process. He tells Alaizabel that she and Thaniel must finish the job.