In adulthood you have an encounter with an unusual person, who as far as you can tell, has no home or family. You offer to help in anyway you can. After accepting your hospitality the stranger vanishes leaving a note "Your kindness will be remembered".
Social. A social enemy might be a personal rival, an idealogy, or a cultural trait. Its an enemy tied to interactions with others, they may have trouble fitting in, be a social outcast, not identify with their culture, or be wanted but their own kind.