How might someone who acts kindly be described?

The word that best describes someone who acts kindly is "compassionate." This term specifically refers to having a deep awareness of and sympathy for the suffering of others, coupled with a desire to help. Compassion involves not just a friendly demeanor, but a genuine concern for others' well-being, which aligns closely with the concept of kindness.

The other options, while they may convey positive qualities, do not capture the essence of kindness as effectively. Being "friendly" generally indicates a pleasant and amiable demeanor, but it doesn't necessarily imply an active concern for others' struggles or feelings. "Sincere" reflects honesty and authenticity in one's feelings or actions, which is important, but it may not relate directly to the notion of kindness in the context of assisting others. "Affectionate" pertains more to showing affection or warmth, often in a physical or emotional sense, but again, it doesn't encapsulate the broader scope of kindness that involves empathy and action towards alleviating others' difficulties.

Thus, "compassionate" is the term that most directly relates to kindness, defining not just a characteristic but an active engagement with the feelings and situations of others.