Who was Hagar, and what made her so special that she received a visit from the Angel of the Lord?
Hagar was Sarai's handmaiden, or slave. She had no personal rights as is shown by this narrative from Genesis 16. Sarai was unable to bear children, so, like we so often do, she took matters into her own hand. She decided that if her handmaiden bore a child, then she could claim this child as her own, thus solving the problem of her own barren condition.
How often do we do the same thing? God does not answer our prayers in the manner in which we believe He should, so we determine to solve the problem in our own way.
However, as is also the normal case, nothing went the way Sarai imagined it would. Hagar did, indeed, become pregnant by Abram, but now Hagar looked down upon Sarai. She may only be a slave, but she was able to do what Sarai could not--become pregnant by Abram. She was no longer the diminutive, submissive slave.
When Sarai brought her complaints to Abram, he told Sarai to do whatever she chose with Hagar. Sarai, in retaliation, treated Hagar harshly.
One has to feel a great deal of empathy for Hagar. She chose none of this on her own, and now she was being treated harshly by her mistress. She did, again, what we are so often prone to do--she ran away.
And, that is where the Angel of the Lord found her. And the angel of the LORD found her by a fountain of water in the wilderness, by the fountain in the way to Shur. Genesis 16:7