white knight
1. One who saves another person from harm or a difficult situation. I don't remember much from the accident, but I am forever grateful to the white knight who freed me from my car. When Sally came to help with the kids when I was in the hospital, she really was our white night.
2. A person or company that purchases, or gives money to, another company that is in financial distress or about to be acquired by a third party. We all thought we were going to lose our jobs, but then a white knight purchased the company and saved it from bankruptcy.