Kind-heartedness is the quality of being kind and generous — the warmth in a person that makes them help. You can talk about someone's kind-heartedness the way you talk about their patience or their courage: it is a way a person is, not a single act. Though actually, everyday English often just says kindness. Kind-heartedness is a little more specific: it points to the heart, the feeling, not only to kind acts. It is a warm, slightly old-fashioned word, more at home in a story or a thank-you speech than in a text message.