The Day Boy and the Night Girl by George MacDonald

The hero of the sun-drenched hours, he looks godlike with his golden hair aglow in the sun as he hunts the wild beasts of field and wood. Yet as the light flees at the onset of evening, his self-assurance and bravado fade with the light . . . and he finds himself beset by terrors he cannot reign. Then he discovers solace, in that terrifying night -- for within the depths of his fears he encounters a strange, beautiful creature who offers him comfort -- a girl, who is as much a creature of the dark hours as he is of sunlit days. Watching over both their lives is red-haired witch Watho -- who with guilt and malice in her heart first cares for the two youths -- then jealously plots their destruction!