What adjective describes someone who is unfeeling and hardened in nature?

The adjective that describes someone who is unfeeling and hardened in nature is "callous." This term is often used to characterize a person who shows a lack of empathy or concern for others, indicating a certain emotional insensitivity or cruelty. A callous individual may disregard the feelings or suffering of others, showcasing a heartless disposition.

In contrast, "candid" refers to honesty and straightforwardness, often in communication, and does not imply any hardness of heart. "Benign" denotes a kind and gentle nature, suggesting an inclination toward being harmless or beneficial, which is the opposite of unfeeling. "Cantankerous" describes someone who is irritable or difficult to deal with, often characterized by a bad temper, rather than a total lack of emotional response. Thus, the choice of "callous" accurately captures the essence of being unfeeling and hardened.