Question. Context: Ten years before the events of the series, Theon was taken hostage by Eddard Stark. He would have been executed had his father Balon displeased King Robert Baratheon. Theon was raised at Winterfell with the Stark children and developed a close friendship with Robb Stark. As an adult, Theon was arrogant, cocky, and proud until being imprisoned by Ramsay Snow, who is the eldest and bastard son of Roose Bolton. During the imprisonment, Ramsay severely tortured Theon physically and psychologically before emasculating him and mockingly changing his name to "Reek".

Question: who is the man who tortures theon greyjoy?
Answer. Ramsay Snow
What step-by-step reasoning justifies that answer?
Reasoning: The relevant information to answer the above question is: During the imprisonment, Ramsay severely tortured Theon physically and psychologically before emasculating him and mockingly changing his name to "Reek".