Diehard

He's just a diehard company man.

She seems to be a diehard fan of this author. She even has his anthologies where all he did was collect short stories.

Tom is a diehard Yankees fan, and rarely misses a home game.

I'm a diehard co-op member.

For a Roman Catholic teacher, he sure is a diehard fundamentalist.

The libertarian right of the Consservative Party, many of them diehard Europhobes, privately referred to the health minister and his advisers as "the Gestapo" for their faith in enforced lockdowns.

I should like my Scottish correspondent to see recent statements by leading officers of certain American railways which both have been among the most extreme diehards for steam, and also have developed the most powerful and outstanding steam locomotive designs—[…].