Die-hard

She's die-hard coffee drinker.

Sami was a die-hard Christian.

For die-hard advocates of free markets and limited government, the abrupt change in direction is horrifying.

And the kaleidescope ^([sic]) of distinctive colours sported by the spotlessly-maintained diesels of many different railways in the great Union stations of cities like Washington and St. Louis must have some thrills even for the most die-hard steam enthusiast.