Opposite words for callous:
kind, fond, softhearted, gentle, kindhearted, mild, warm, affectionate, humane, merciful, benignant, tender, good-tempered, loving, indulgent, friendly, warmhearted, cordial, tenderhearted, sensitive, compassionate, sympathetic, untoughened, good-natured, charitable, lenient, gracious, kindly, understanding, clement, benevolent, tolerant.
Usage samples:
  1. They had been for so many years in contact with misery in its worst shape that the last spark of human feeling had died out in their callous hearts.
    "A Narrative of Captivity in Abyssinia With Some Account of the Late Emperor Theodore, His Country and People", Henry Blanc.
  2. At the callous cruelty I stood for an instant petrified; I heard Drake swear wildly, saw his pistol flash up.
    "The Metal Monster", A. Merritt.
  3. " He-" she murmured, for it was so difficult to speak to this callous wretch, who was obviously mocking her in her misery-" he- you- you are not keeping him in irons?"
    "El Dorado", Baroness Orczy.
  4. His early struggles had made Mr. Appel callous.
    "The Dude Wrangler", Caroline Lockhart.