white knight

Short definition from Wiktionary

Noun

1. 1
   An individual or corporation that intends to acquire another company in order to avert a hostile takeover.
2. 2
   A hero, savior, or righteous individual.
3. 3
   Someone who unnecessarily defends someone else.
4. 4
   Someone who unnecessarily defends someone else: A man who defends a woman in debate etc. in an attempt to gain her favour.
5. 5
   A mushroom of species Tricholoma album.
6. 6
   Used other than figuratively or idiomatically: see white, knight.
7. 7
   Alternative form of white knight.

Verb

1. 1
   To unnecessarily defend someone, especially in an attempt to gain favour.
2. 2
   Alternative form of white knight.