To be stalwart is to be loyal, no matter what, like your friend who remains a devoted fan of an actor she's admired since childhood, even if that was the last time the guy made a decent movie.
Stalwart can describe someone who's able to keep on going even when things get hard, like a marathon runner who doesn't slow down, even after spraining an ankle, or a supporter of a political cause that everyone else has long declared over. In U.S. history, the word stalwart was used in 1877 to describe Republicans who remained unwilling to trust the South, even though the Civil War was long over by that time.
