A warm-hearted person is kind in a natural, feeling way — they care about people, they notice when you are unhappy, and they want to help. It is an adjective, and it is always a compliment. The key thing is that it describes a person's whole nature, not one action: you are warm-hearted the way you are tall, all the time. So it fits people best ('a warm-hearted old lady', 'she's a warm-hearted boss'), though we also stretch it to things that come out of that nature — a warm-hearted welcome, a warm-hearted letter, a warm-hearted film. Compare it with its neighbours: 'kind' is about what you do; 'friendly' is about how easily you talk to people; 'warm-hearted' is about the feeling underneath, the sense that this person genuinely wishes you well. It has a slightly old-fashioned, story-book flavour — you meet it in novels and in speeches more than in a text message to a friend. Its opposite, cold-hearted, is much stronger and much crueller than warm-hearted is sweet.