An ascetic is one who practices extreme self discipline and abstention. Frank Castle definitely fits this bill nicely. If you look into the word’s origin, you will see that it comes from the Greek word askētēs. Askētēs is Greek for one who exercises.
Frank Castle has never given up his military discipline. Whenever, he is not on mission, he is getting ready for the next one. As you could expect, for a hero without super powers, that requires constant physical training. How else could an ordinary man be able to handle himself against super villains or legions of mob goons?
The Punisher is a man that is perfectly content eating MREs and living without material comforts. Until recently, he wasn’t much for conversation or even the comfort of the opposite sex. His mission to avenge his family by punishing criminals could almost be described as spiritual. In the Nam series, a supernatural voice calls to him on the battlefields of Vietnam. However, whether this voice was good or bad, it seems Frank Castle has tamed it for his own purposes.
The pious nature of Frank Castle begin even before he became the Punisher. As a young Frank Castiglione, he was the son of Catholic Italian immigrants and thought about becoming a priest. However, he ditched seminary school for the military after being unable to forgive sinners. In Marvel’s Age of Apocalypse, an alternate but still ascetic version of Frank Castle exists. Instead of becoming the Punisher, he chose to become a monk, Brother Frank, and offered sanctuary to humans and mutants alike.
Nowadays, Frank Castle’s asceticism has been toned down quite a bit. In the last few years, he has even hooked up with a couple of ladies, including Electra. He even breaks from his MRE routine to eat at the Nickel Diner, somewhat routinely. While there, he even strikes up conversations with the cops who eat there. Although, a little less ascetic, he still has the willpower to resist one of the place’s trademark bacon maple donuts.