diehard

Short definition from Wiktionary

Adjective

1. 1
   Unreasonably or stubbornly resisting change.
2. 2
   Fanatically opposing progress or reform.
3. 3
   Complete; having no opposite opinion of anything in a particular topic of one's values; thorough of in one's beliefs.
4. 4
   Strongly or fanatically determined or devoted.

Noun

1. 1
   A person with such an attitude.