10. Who is the leech? Why is he called that in the title of the chapter?

Chillingworth is the "leech" a common term given to doctors who, at this period of time, often used leeches to bleed their patients to bring them back to health. Since Chillingworth is from Europe with doctoring skills learned there and is also knowledgeable of doctoring in natural arts, the title of "leech" fits him even more than other doctors.