This question comes right from the pages of the Bible. In the record of his interaction with a certain man, Jesus, after summarizing all of God’s law in two great commandments; love God with all your being, and love your neighbor as yourself, is asked this very question, “and who is my neighbor?”
Jesus answers with a story about a man who was attacked by robbers along the road and left for dead. Three people of three different social classes then happen upon the man. Only the third stops; a Samaritan, the last one the hearers of the story would expect. He binds up the man’s wounds, takes him into town and provides for his care.
The point of the parable, and the answer to our question is that anyone of genuine need whom God puts in our path, that we have the ability to help, is our neighbor, even one whom we might otherwise consider an enemy.