mercenary

Short definition from Wiktionary

Noun

1. 1
   One motivated by gain, especially monetary.
2. 2
   A person employed to fight in an armed conflict who is not a member of the state or military group for which they are fighting and whose primary motivation is private gain.
3. 3
   One hired to engage in a figurative battle, as a corporate takeover, a lawsuit, or a political campaign.

Adjective

1. 1
   Motivated by personal gain.