white knight

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One that comes to the rescue; a savior.
• noun A person or company that rescues a targeted firm from a takeover attempt by buying the firm.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun business An individual or corporation that intends to acquire another company in order to avert a hostile takeover.
• noun fiction a hero, savior, or righteous individual

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a company that is a friendly acquirer in a takeover